Honestly this post reminds me that there a lot of people using the revolutionary language and history of Marxism to justify their own nationalism or other views which are not those of internationalist Marxism or Communism.
Yes religion is idealistic. Idealism in the history of Marxist use can mean both ontological idealism, in which one thinks that the only or most fundamental components of what exists has the basic properties of the mental; and epistemic idealism, where the explanation of certain events or properties is based first and foremost on ideas, in which ideas are given priviledge of explanation, rather than what those ideas are ideas of. Someone can be ontologically idealist on paper while acting like an (epistemic) idealist in their reasoning, of which many liberals are perfect examples. Of course, for a genuine materialists, ideas do also in a sense reduce to matter, while also emerging out of matter, and so are types or expressions of new properties which emerge out of very complex forms of matter. You do give a good example of epistemic idealism, which is people ignoring the historical and more specifically the religious context of these countries in which national liberation movements are active. However it is equally idealist to ignore the profoundly reactionary implications of a group being Islamist (as opposed to, say, simply being a group most or all of whose members are Muslims). I don't think anyone is implying that the political role, influence or importance of religion is to be ignored here. If anything it is the opposite. Though it seems you are implying they are in order to imply their view of what that significance is is incorrect, contrary to your own I'm guessing.
You are also introducing an unjustified exclusivity between (1) Islam being idealist (which you haven't made clear what you mean), but is most definitely when we are talking about the ontological content of Islamic beliefs and the kind of religious reasoning that is based on it; and (2) westerners not understanding the 'unit of opposites', which seems to be to just be a force of relativist mysticism; if you mean that is no such thing as truth or a correct view. This is not Marxism (a modernist ideology). This is simply another expression of postmodern relativism. If here you mean that have to recognize that there are potentially progressive aspects with non-preferable components such as strong religious ideology, then sure, that is something to be recognized. Islam is a material phenomenon, like every other religion, because it exists in the social world. But the actual beliefs, the content of it, are not materialist. Materialism is not the same thing as something being material. This is just a confusion of what different words mean.
An irony is that you yourself are essentializing Islam (or maybe it only seems so due to the lack of clarity in what you saying) by claiming that it 'is the form which the anti-imperialist struggle takes form'. Reactionary groups can oppose Imperialists. This is not a mystery. And assuming that because they do that they must be progressive is precisely the kind of 'absolute, black and white' mystifications you are accusing others of committing. There are plenty of non-Islamic, non-religious liberation struggles, and again there is a great difference between a movement containing religious people, and the movement being religious in character. Every Islamic or Islamist revolution has been a reactionary nightmare, for the people in general and the left in particular. The vast majority of genuinely progressive and successful revolutionary movements in recent history have been my
Marxists in many non-Western societies are often far more explicitly anti-religious in private than the sheepish left of the West, who are terrified of losing their virtue-signalling points. One example is Haiti: when I've spoken with Haitian comrades, they are fully conscious of the reactionary potential of religion, because they are in a society in which religion (whether in the form of Christianity of Voudou) is an immense obstacle and impediment for communist education, radicalization and organization. We are not going to bring people into the communist fold by ignoring reactionary views they hold or not attacking them. If anything, the fetishism of Islam extending into one of Islam, on a supposedly Marxist forum no-less, is an expression of the fact that these individuals actually almost certainly have no experience of organizing politically as Communists, let alone in the countries they are talking about.
Islamic socialism is certainly better than no socialism at all, and you are completely correct that white western atheists absolutely and completely condemning these movements purely on that basis is chauvinistic and ignorant, that is nevertheless completely irrelevant to any discussion of the political nature of religion, and there are certain fairly unavoidable conclusions on that front when we scientifically analysis the historical and contemporary evidence as Marxists. It's not clear to me how anything you'd said impacts in any way any serious discussion of the political nature of religion in general and particular religions specifically. Not all religions are equal, but there are sufficient similarities (hence using the common term 'religion') for us to be able to start making more general theorizations and conclusions about it. This includes the how religions function politically and influence politics in different contexts.
Another issue here is that no-one is making a distinction between Islam and Islamism, which is particularly ironic in a thread with a couple self-flagellating white westerners virtue-signalling other their desire to understand the religion of the downtrodden and avoid Islamophobia.
Regarding this:
"In my eyes, the answer is simple. It is because the Western left still carries the mental burden of colonization, of cultural genocide, and they project it onto the global south - onto the ummah."
It seems equally obvious to me that this is a giant jump in reasoning. I'm not really seeing the evidence here. This is also, again, ignoring the massive elephant in the room of Islamism. Fear of Islamism is the most rational emotional response to have towards it. Anyone who says otherwise has not lived in Islamist societies, does not understand Islamism and its both its differences with and intimate connections to Islam more broadly. This whole discussion is also again a reminder which angers me immensely that most Marxists from these parts of the world are not having their views discussed very clearly, and is ignoring that the vast majority of communists who have lived under Islamism understand that it is an extremely reactionary ideology that makes life misery. By far the most anti-theist people I have ever met are my Communist friends and comrades from and in the Islamic world, and most of all the Iranians. You seem to me to be giving a clearly idealist explanation here. The form of anti-imperialist politics is deeply influenced Islam and Islamism because these are deeply religious societies in which secularism and notably the secular left failed, and because they are responses to the correctly perceived, widespread racism and Islamophobia of the West, because many people in their suffering, misery and alienation turn to religion as a consolation that becomes essential and precious in their lives, and because modern Islamism explicitly formed itself on a conception of politics similar to the Leninist clandestine party organization, aiming for mass radicalization where they would take advantage of the radical energy of mass movements and direct them for reactionary ends, very similarly to Fascism.
On the France point, which I can speak to as having lived there, it is correct that the form of 'secularism' practiced as policy by the French government is not only inconsistent in its application to Muslims compared to Christians, and thus does not live up to the ideal of secularism which should be aimed for, but is deeply and structurally discriminatory in its application. Going from the fact that French secularism is racist, to the conclusion that secularism is racist, is like realizing that a square in front of you is red, then seeing a red circle, and saying that the circle is square. The French state is racist, but that does not imply in any way that we should not be secularists in our policies. Religious justifications have no place in a Communist party. Period. End of discussion.