Speaking from personal experience, I think the problem is that the 'argument' for Holocaust denial can be made to be extremely compelling for those who don't really know how historical evidence works. I saw a video by Anthony Lawson which was (I now know) the usual HD trick of repeating things like, 'There still isn't one single piece of physical evidence that shows...' etc. If you don't know about consilience (as I didn't), then that sort of rhetoric is extremely powerful.
I was never actually convinced that there was anything in the claims because I immediately did a google search on them (and as I'm sure you know this very quickly leads to them all being comprehensively debunked), but the fact is after watching that video I did think, 'Wow. Maybe there's something to this after all - maybe it is being suppressed' etc. and that kind of casual, lazy 'questioning' of the Holocaust will be what surveys like that are measuring: the results of idle office conversations with someone who saw something on twitter and says, 'Did you know that the wooden doors at Auschwitz couldn't have been used as gas chamber doors? That 'six million' number was changed from 1 million overnight. Zyklon B can't actually be used to kill humans. There weren't any holes in the roof. I know! I couldn't believe it either!' and so on.
The problem is that the deniers know their market and know that saying, 'There isn't ONE SINGLE piece of evidence that directly proves the Holocaust happened' will convince a lot more untrained people than saying, 'There are a million things that indirectly prove it happened'.