And it seems to be mostly criticizing a version of sociobiology that attaches changes in post-Paleolithic to changes in genetic basis of humanity, and existing variety of behaviour to existing variety of genes in modern individuals.

Where are you getting this? You go on to mention their complaint #5 that they don't like Wilson's reconstruction of the ancestral environment, which makes it sound like they're saying that he believes in uniformity.

They do say that some people say "poor people have bad genes" and they fear that those people will turn to Wilson, but they do not say that Wilson claims that. They talk about two forms of "biological determinism," contrasting Davenport, Jenkins, and Shockley as eugenicists / people who believe in diversity against people who make uniform claims about humanity, based on evolution namely Wilson, Lorenz, Ardley, and ... Spencer. OK, where they chose to put Spencer (who subscribes to both) speaks volumes, but there's a reason they don't come out and say it.

There are some places that could be read as saying that Wilson believes in diversity, but none of those places are very clear. The clearest is the part about homosexuality: surely, no gene for homosexuality is fixed, at least not fixed on! But they follow that with "for Wilson, what exists is adaptive, what is adaptive is good, therefore what exists is good" which makes it sound like he's talking about uniformity, as with "conformer genes" at the beginning of the paragraph. Anyhow, Wilson responds that he explicitly warns against the naturalistic fallacy and that the whole thing is a gross misrepresentation.

Nor does the letter seem to portray Wilson like Spencer, a believer in rapidly changing uniformity.