Why not just say "we ban speech that says the Holocaust didn't happen"? Why get it classified as hate and then because somehow hate is censorable get it autocensored? It seems somehow disingenuous.
The only reason people deny it is because of anti-semitism.
What does the word "hateful" mean? The old meaning is "full of the emotion of hate". Someone who thinks the Holocaust wasn't real could in theory have no strong feelings about it and think it has no relevance to their lives.
It is not required by definition that Holocaust denial is hateful (using traditional definition of the word "hate"). Nor is it required by human psychology (for example, you could have someone who read an unfortunate sampling of books as a child and took "disbelieve anything the victors of a war say about their enemies" as gospel and never got educated on the details).
I would really recommend doing a cursory, bare-minimum reading of the associated Wikipedia page [0] and citations. Plenty of historians revise the events surrounding the Holocaust to provide less biased and more nuanced information. Very different from taking an assumption as fact (the holocaust did not happen) and working backwards from that.
[0] https://en.wikipedia.org/wiki/Holocaust_denial