True, though I wouldn't deny him a place among "actual historians" or be dismissive of him. His work has been very influential, even if it doesn't turn out to be the authoritative explanation it appears to be.
Funny, just a propos of this, I just finished Kyle Harper's Plagues Upon the Earth, which is a sweeping history of human disease, from the origins of humanity to the present. Fascinating read and very illuminating, though with all macro-narratives of this scope, one always wonders how well it will stand up to scrutiny. Either way, I learned a lot. For instance, I had heard before that people with ancestry in certain parts of Africa are prone to sickle cell anemia, but I never knew why. Turns out that malaria is massively endemic in that part of the world, and the malaria microbe feeds on human red blood cells and can be very lethal. Anemia, even though it's an illness that weakens a person, gives them a better chance of surviving malaria by starving the virus, so it's adaptive to the disease environment.
Another interesting one was polio. My mother had it as a little girl in the 50s. It seems that, ironically, the polio epidemic of her generation was actually a product of better disease control. In an earlier era, children had generally been exposed to it as babies, when they were still protected by their mothers' antibodies in breast milk and thus developed an immunity. With greater sanitation and hygiene they no longer got that exposure so early, and the way the disease works, the later in childhood you're exposed to it, the worse it is. My mother was 11 and it paralyzed her from the waist down for a time.
As far as Diamond goes, a central idea of Guns, Germs, and Steel is the "germs" part. In this narrative, when Europeans cross the Atlantic, they bring with them a whole battery of lethal diseases that they're resistant to and the natives aren't. They have all these diseases because they developed agriculture earlier and there are lots of zoonotic microbes they get from their farm animals. These diseases then ravage the Native American population without the Europeans even really understanding how, thus paving the way for conquest and colonization.
According to Harper, though, this doesn't really take account of the way these diseases actually work. There ARE zoonotic diseases (ones that originate in another species and make the jump to humans), but that's only one category of several types. Perhaps more importantly, when you look at the records, the European colonizers are getting ravaged by disease as much as the natives are. Famously, half the people on the Mayflower don't survive the first year. Colonization was actually deadly to the colonizers too. Also, as Covid taught us, diseases themselves have dynamic histories; new strains emerge at particular points in time and turn out to be more or less virulent than their ancestors. It takes several thousand years for humans to evolve an adaptation to resist a disease as a population, as happened with malaria in Africa; But most of the other diseases involved in the Columbian exchange were more recent than that. People only become immune after they personally have been exposed to it, not by ancestry. As anyone who knows the Edward Jenner story should know, Europeans didn't have some innate immunity to smallpox in Columbus' day (nor for centuries after). It killed them too. (A bit weirdly, too, plague was not among the diseases exported to the Americas, even though it was still affecting Europe all through the early modern period).
So the story is not as simple as Old World diseases kill off New World population, immune Old World colonists sweep in to take over emptied-out land. The use of violence in conquest is also critical. What seems to have happened, in Harper's analysis, is that normally after a lethal epidemic, populations recover; people have more babies, and life as usual resumes. But in colonial settings, this didn't happen, because European action inhibited it among the natives. People were driven off their land or forced into slave plantations or labor in gold and silver mines. Harper doesn't go there, but it occurs to me that alongside the trio of the title, Diamond had talked about European literacy as another asset enabling their conquests. He was probably right, but not so much for the reason he suggested, that it made Europeans craftier strategists and more alert (or prone) to treachery. It may have been the application of tools of state control, of bureaucracy and regimentation, that enabled them to prevent population recovery and thus end up dominating in the long term. It also drove propaganda about the New World that recruited more and more adventurers willing to risk death and disease to make their fortunes overseas. Otherwise the early modern European intrusion into the Americas might have been as temporary an episode as Leif Ericsson's.
