Of course. The Nazis accused Jews of committing genocide against white people. That's why the 14 words are a thing, "We must secure the existence of our people and a future for white children," the implication that "the existence of our people" is under threat. It's the same reason modern facists chant, "Jews will not replace us." Nazis (not just the modern ones, but the OG ones) promoted conspiracy theories that Jews were trying to make white people become gay/trans to prevent them from reproducing, and communism was supposedly another Jewish plot to make white people too soft and tolerant to use force against Jews. And naturally, interracial marriages were another form of "genocide," as they diluted the racial purity of of the Aryan race.

I have used the argument in the past, “If given all available evidence you’re still not sure a genocide is happening, you can probably safely assume that it is, because humans just constantly do genocide.”

That's a bad argument regardless, because the default position on any claim or accusation should always be skepticism. The burden of proof is on the person making the claim. Some people seem to have gotten it into their heads that accusations of genocide are an exception to that, somehow.

Generally, historical genocides such as the Holocaust are incredibly well-documented, and the term "genocide denier" is meant to apply in such cases. There is nothing morally wrong with denying a genocide that doesn't have the same kind of evidence backing it up, that's just called due diligence.