Holocaust denial is a pro-genocide ideology. To deny the holocaust of the past is part of denying violence in the present day. It also implies that Jews are liars - it's a very strong piece of anti-semitism.
Holocaust denial is not something people come to accidentally or through a rational evaluation of the evidence. It's something people reach through anti-semitism. What do you think the reactions of someone denying the holocaust in the past would be to evidence of more genocide being planned in the present or future? Deniers are signalling their willingness to help genocide by turning a blind eye to it. They're showing that fear or hatred of Jews is more important to them than facts.
Can you make this case more clearly? I get that it's anti-semitic (the lying point) but not how it's pro-genocide.