This is in fact not true; we've evolved the persistence of lactose metabolism into adulthood at least twice, and both times at least in the neolithic.
>If this be the case, then it is certain that no perceptible evolutionary changes in human nature can possibly have taken place within the few thousands years of which we know or can conjecture the history.
From my understanding, I believe Huxley is talking more about the ethical or moral characteristics of human nature. Greed, lust, selfishness, etc. rather than the physical makeup or allergies of humans.
Additionally, even if he had, it's the exception that's so minor it proves his point - while lactase persistence evolution is a really cool bit of trivia, it's also clearly a really, really minor contribution to "human nature". We don't regard peoples as significantly different today even when there are numerous measurable differences, after all - we regard stuff like that as fairly trivial, right?
Finally, apparently this most recently was an impactful evolutionary advantage 7500 years ago - but that could be consistent with his perception of the lack of changes to human nature within the few thousand years he knows.
However, as a critique of Huxley's piece, I don't it's really relevant. If you're talking about how progress is essentially purely cultural (as he claims), and not down to intrinsic biology, then cool bits of historical trivia aside, this just doesn't quite seem to cut it.
The context being stuff like...
> Beneath the shifting surface, human nature remained substantially the same.
Human culture clearly isn't so dramatically different in places where this did not evolve, and what differences there are are certainly down to many factors, of which lactase persistence is just one.
Also, consider that he says
> A man born in the twentieth century A.D. has no better chance of being congenitally intelligent and virtuous than one born in the twentieth century B.C.
or that
> Evolutionary progress of the species has not been perceptible within historical times and may, for all practical purposes of record of prophecy, be left out of account as non-existent. Uninheritable progress, due to tradition, has genuinely taken place in the realism of science and technology, where each worker stands on the shoulders of his predecessors.
...he's not disputing the dramatic impact of evolution per se; simply that on the time scale of cultural evolution, the genetic variety is so slow as to be largely irrelevant.
He also says a bunch of other stuff; but I think the observation that the literally incredible progress (I doubt anybody in e.g. ancient Rome might have foreseen the average life we live today) humans have made over the past few millenia is down to memes, not genes.
Genes are still important; but they don't explain progress.