The Left denies genetics in the same way The Right denies climate change, and for the same reasons. And any ideology which must deny science in order to survive is inherently broken and likely dangerous because reason was not used to get there.
In this case, I think Mosimann is the one abusing the science. He asserts that genetics accounts for 20-30% of the black/white race gap, but there's no scientific basis for that. Just because there is some genetic component to IQ measurements - as found in twin studies - this doesn't say anything about how much of the black/white IQ measurement gap is genetic compared to environmental. In general, it is really difficult to tell the difference between what is hereditary versus what is environmental - because humans are never raised in controlled scientific conditions. Twin studies are essentially the only good data on this. However, they only tell about genetic variation within a family. It doesn't tell about the variation between different populations.
It is entirely possible for there to be 20% variation due to genetics overall, but a particular difference between population A and B is 0% genetic, and the difference between populations B and C is 100% genetic. For that matter, there is no reason to think that environmental and genetic differences go in the same direction between two populations. In other words, maybe Asian genetics are predisposed towards *lower* IQ scores - but environmental factors are greater than those genetics, making score higher.
I think it's a common assumption that genetics will exactly mirror environment - so any population difference is caused by a mix of matching genetic and environmental factors. So, if blacks are good at dancing, then maybe it's 30% because of good dancing genes, and 70% due to culture. But in all likelihood, genetic effects are going to be completely independent of cultural effects.