The issue is that the "young" demographic mentioned in the article is ages 18-29, and high school must have changed quite a bit in recent years if students have not actually heard about the holocaust or WWII.
Well, in my (Gen-X) high school history classes, we only got to WWI (and just after like the 1920's - 30's). I always thought that there is this weird assumption that things that happened in the teachers' lifetime or just previous are "already known" or that the culture will just implant that knowledge somehow.
Same here (also Gen-X). Here in Canada I never learned about WWII in school. I barely remember learning about WWI, and that was usually just around Remembrance Day (celebrated on the anniversary of the Versailles Treaty).
We learned a lot about early Canadian settlement and relations between Europeans and First Nations. By the time I found myself in high school we only needed to take a certain amount of "social studies" credits and, while history was included in that category, there were options that were not specific to history. My experiences learning about Canadian settlement had been so dry and boring that I had made up my mind that I didn't like history at all and so I made every choice in high school to opt out of having to take history classes. In other words, WWI and WWII, if they were taught at all were "optional."
We'd have to compare against a similar poll from decades ago to figure out if this true—it could be it used to be that 40% of kids didn't believe in it.