Impeccable reasoning on your part. I threw the book at a wall after a few chapters when I saw where it was going.
I believe Diamond's distinction in "Guns, Germs and Steel" was in the herd nature of the animals. Ie Horses will follow their herd "leader" while Zebras will not follow the herd leader but resemble an pack of indpendant animals more. Humans can domesticate the animals that then follow them as the herd leader.
I agree Diamond is a lib. But His book does look at a larger system view of why some cultures dominate and some get wiped out. It's a good read for those interested in looking at how weapon/technology was passed/stolen among cultures. He discusses why those with domesticated animals develop immunities to germs that wipe out other cultures. Of course he missed the larger picture that those Countries that follow God and promote Freedom are More "inspired" and develop quicker. So his book is flawed in not acknowledgeing God. But as for analyzing the underlying processes that Plant and Animal domestication and transfer of technology and language among peoples, it's OK. I really thought it was a good read, and I'm a strict constitutionalist politially.
He has been criticized by the Green/Eco movement quite a bit too for the work he has done with Oil Companies, he talks about it in his next book, Collapse.