(Yes, it's interesting that the article calls the Holocaust "one of modern history’s greatest crimes" - but gives zero words to any of the others. And there's no hint that the poll asked about any of the others.)
If your knowledge of history is pretty minimal, but you keep seeing "Because Holocaust!" used as some kinda magical rhetorical/moral trump card...then having serious doubts about that "Holocaust" thing is fairly reasonable, AS AN UNINFORMED EMOTIONAL REACTION. [Please re-read the shouty part there 3 times, before you angrily respond.]
Vs. if you do know the history - the Holocaust's "special cultural/moral status" still really sticks out. My pet theory is that, from the PoV of well-educated, middle- and upper-class western whites, the Jewish Holocaust victims "look" like just-one-thing-different-from-me analogs. Vs. the poor Slavs, Romani, Black, etc. victims do not - so killing "their kind" in 7-or-so-digit quantities just doesn't push the same deep emotional buttons.