While agriculture has had health-effects, Jared Diamond is a quack who writes shallow fluff-anthropology and seems to take the approach of deciding on a conclusion, then go finding research to justify it.
While I'm with you on Jared Diamond not being the best scientist, your criticism misses the mark. What you've described is part of the scientific method: you come up with a hypothesis and you test it.
A better criticism of Jared Diamond's work would be that his work suffers from a lack of primary source evidence: he's not doing experiments or directly synthesizing actual data, he's synthesizing the results of studies. And since there's no objective way to choose which studies are relevant or evaluate the methodologies of different studies, that inevitably introduces his own bias into the process.
That said, I haven't seen any studies contradicting the claim that grains in diet are correlated with tooth decay (which is a smaller claim than the larger claims Diamond makes about health, and which I think the primary studies he cites actually support).
A better criticism of Jared Diamond's work would be that his work suffers from a lack of primary source evidence: he's not doing experiments or directly synthesizing actual data, he's synthesizing the results of studies. And since there's no objective way to choose which studies are relevant or evaluate the methodologies of different studies, that inevitably introduces his own bias into the process.
That said, I haven't seen any studies contradicting the claim that grains in diet are correlated with tooth decay (which is a smaller claim than the larger claims Diamond makes about health, and which I think the primary studies he cites actually support).