This is the logical outcome of a certain party both pushing an identity of male-dominance based on toxicly masculine characteristics, and who also cozies up to White Supremacist orgs like the Proud Boys.
The GOP is and has always been a fertile recruiting ground for white supremacist and neo-Nazi groups, and the "manosphere" has accelerated the spread of their core rhetoric (which includes Holocaust denialism) geometrically.
The claimed figures are arguably too clean. Wouldn't be surprising if it was a bad/fake poll. If 1/5 of young people were really Holocaust deniers, we'd absolutely hear about those types of people all the time.
I was educated in the US public school system in the mid 2010s and I felt like history class was 1/4 American slavery, 1/4 trail of tears, 1/4 revolutionary war, and 1/4 holocaust across middle and high school. Apparently that’s not normal, or the other kids weren’t absorbing anything.
The world is not getting more stupid, cross-generational memory has always been flawed. If we want to give new generations the tools to build a better future, we either need to teach them to learn from our collective experience (history) or to think of consequences ahead of time (futurology and ethics). Otherwise they are doomed to repeat the same mistakes.