"Debate" implies opponents, with fixed positions squaring up to each other, with the expectation that one side is going to win and the other lose. It's a kind of game. It's perfectly possible to have a debate in which neither side is right.

"Discussion" implies an exploration of a topic in the hope of getting a clearer understanding of the truth.

Clearly anyone who denies that there was a sustained attempt by the Nazis at genocide of Jews and Roma, and mass killings of other categories of "undesirables" is not merely mistaken, but perversely mistaken. The right approach to that kind of thing is not to "debate with the deniers", but to present the evidence that disproves the assertion. The aim is not to convince the people making the assertions, but to make it harder for anyone to be taken in by them.

However that shouldn't mean that a particular account of what happened should be seen as set in marble, not open to review in the light of subsequent information. Nor should it mean that the way in which the Holocaust and accounts of the Holocaust have been used subsequently should be a no go area.