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 doubt it. Probably has to do with incomplete/bad education. I imagine we would average similarly badly when asked about other WWII era trivia.

Here’s an article on Holocaust knowledge across the US from 2020: https://www.pewresearch.org/religion/2020/01/22/what-americans-know-about-the-holocaust/

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.