They don't deny human biodiversity because they think it's false. They deny it because they think it helps justify an "oppressive narrative". It's all feelings and "oppression". If some groups are less intelligent on average, and this is due to natural reasons, or in fact any justifiable reason (natural or artificial), then those groups will be justifiably lower on the hierarchy than others. From their perspective, hierarchy is intolerable (and cannot / should not be justified) and so anybody sharing those facts must be "complicit" in what they call "white supremacy" (they call it this even if Asians are shown to have the highest IQ).

Then there are the "useful idiots" who believe the lies the former group tell about human biodiversity not existing. They truly think the Critical Social Justice types are simply complaining about unfair treatment, about prejudiced cops, and that we should strive to erase prejudice, and that we really are all the same underneath. They couldn't be further from the truth. Critical Social Justice activists do not care that human biodiversity exists, they simply want to destroy the narrative that it does. They don't care about "prejudice" because they'll happily pre-judge people if it advances their agenda. They only care about "oppression" which means when any (racial, ethnic, sex) group has more power than any other - and in particular, since white men have more power than other groups, they will deconstruct any narrative that justifies any of that power, regardless of how true it is.