To be honest I didn't understand why they felt the need to position it as a rebuttal of GG&S when reading it either.
A lot of the complaints about GG&S are to some degree a complaint about Diamond in general being mapped on to the least offensive of his works, and if that is the only exposure you have to Diamond, it can be seem like the complaints are coming out of left field.
Which discredits the entire field. Ad hominem arguments have no place in science. Everyone knows that Newton wrote extensively on the occult but that takes nothing away from his contributions to physics and mathematics.
If I could address his critics as a whole I would say:
Critique the theory on its own, independent of the author. Provide an alternative theory. “It’s complicated” is not a theory.
Furthermore, the kinds of argumements that Diamond would provide weren't actually "tested" per say. Social Sciences are a "Science" (albeit flawed in some shape or form), but are dependent on validating a hypothesis in a reproducible manner as well, hence why economics has basically become applied math since the 50s (and similar changes in other fields like Sociology, Linguistics, Polticial Science, and Anthro as well)
There isn't much difference between grifters like Perun or Zeihan and Jared Diamond.
Also, Jared Diamond doesn't have a background in Economics or Political Science - he is an Ecologist/Environmental Scientist (and one of the best ones at that), and as such reading GGS induces Dunning-Kruger for those with a background in Comparitive Politics.
No one in the academic PoliSci, Sociology, or Anthropology space views him as credible.
He is a great Ecologist and Environmental Historian, but he is not someone with domain experience in Comparative Politics, Political Economy, IR, and/or other adjacent fields which people try to extrapolate GGS to.
I don't have a dog in the fight other than having read (most of) GG&S and found his writing style laborious, but the quoted bit above seems more like an issue with those other people that Diamond.
If I say "x" and people extrapolate that to things for which it was not intended, how does that make _ME_ a grifter?