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.
I love seeing counter-evidence for everything. I estimate that while most of my beliefs are true (otherwise I wouldn't believe them in the first place), a small percentage is almost certainly completely false - and I don't really have any reliable way of telling the two apart.
Indiscriminatingly looking for counter-evidence for all of them can be very rewarding - the ones that are true are much more likely to sustain the assault of it than the ones that aren't. Yes, I might ignore counter-evidence of something that's false, or accept it for something that's true, ending up worse off, but it seems plausible that on average it should improve quality of my beliefs.
For example some of the standard beliefs about human sociobiology that seemed to be extremely widely held here are:
Charting Parenthood: Statistical Portrait of Fathers and Mothers in America disagrees with them.
These are not direct tests of sociobiological claims, so what we have is not exactly what we would like to, but I find them to be quite convincing counter-evidence. My belief in these sociobiological claims is definitely lower than before, at least as far as they concern modern world, even though I can imagine more focused studies changing my mind back.
More counter-evidence for things we commonly believe here, sociobiological or otherwise, welcomed in comments.