Stuff like "there is no written and signed command by Hitler" is really pretty insignificant either way. Neither is the question of precise numbers.

The core fact, proven to the hilt, is that there was a sustained and policy of genocide of Jews, Roma and other people seen as undesirable by the Nazi state. That is the Holocaust, and Holocaust denial is about seeking to deny that.

When the term is used in a much wider sense, to include any kind of speculation or investigation into the details of what happened and how it happened, that plays into the hands of Holocaust deniers.
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The evident self-defeating motive behind the Iran conference is apparently a belief that, if it is accepted the Holocaust happened, this somehow implies an acceptance of the way in which the state of Israel was established, and of the fate of the Palestinians. So Holocaust denial is seen as a way of challenging the right to exist of Israel.

In reality it is the other way round. By linking the Palestinian cause to Holocaust denial the organisers weaken it. It implies accepting the logic by which the Holocaust justifies what happened to the Palestinians.

The truth is the Palestinians were knock-on victims of the Holocaust, rather like driver caught up in a motorway pileup, injured by a vehicle propelled into theirs by another vehicle. No point in denying the existence of the pile-up, and imagining that the primary responsibility for the crash was the person driving the car that actually crashed into theirs.