While I agree with your sentiment, it's not applicable in this context. He's stating a simple factor of most charity work (something I'm familiar with working in the non-profit world when I was younger). If you alienate your donators, you lose their donation. The easiest way to alienate someone is to declare a political stance, and the clumsiest way to do so is to do it by declaring an allegiance to a party rather than describing your support or opposition to policy specifics.
Ideological purity always conflicts with the tactical application of positive change. As an example, what would the US Senate look like if Franken hadn't resigned? What could have been accomplished? What positive changes were prevented? What would the Supreme Court look like now?
Secondly, your hyperbole obfuscates the fact that most Republicans are not pro-genocide, rather, extremists within their party are. Additionally, the identification of Republican or Democrat goes further than political identification in America - it's a cultural identification as well, one that splits along rural / urban lines. I know a number of rednecks from high school who are great guys, shoot their guns, love their gay and brown friends, support abortion, give to charity, and publicly identify as conservatives who hate Democrats... even when on a policy level, they agree with most progressive politics. A big factor in this is the conservative media landscape, which has actively fostered this level of cognitive dissonance, but that doesn't address the question of "how do you convince people to help you do good if they don't agree with your politics?"
Is it better to declare your politics and lose the donations that would allow you to do good?
Or is it better to keep your politics private, accept donations from all comers, and use those resources to make the world a better place?
In my opinion, the best path (and the one Mr. Beast appears to be following) is a middle ground. Don't declare your politics, accept donations, but if a donor has an agenda that conflicts with your politics or morals (like publicizing the donation to whitewash their reputation), reject them on a case by case basis. This lays out your support or opposition in specific instances rather than aligning your actions to the whims of a political party, and thus risk being aligned with the views of extremists within that party.