All you really need is for Pol Pot to become impressed by a tribe which has a strong reverence on hunting while he is fighting in northeast prior to his victory and/or pick up some French philosopher building on Rosseau to argue for a hunter-gather society, and down the rabbit hole we go.
What tribe in the northeast? They're all rice farmers, the nearest hunter-gatherers live in the interior of the Malay Peninsula.
As for the French philosopher angle, that's certainly something Pol Pot could do. People like that exist, but they hang out in anarchist bookstores or live off the grid someplace. Unusually motivated ones might organize something like MOVE. They don't end up becoming the head of state of a country populated primarily by peasants.
As
@HelloThere,
@overoceans, and others have been saying, there's an internal logic to even the most ridiculous and horrible successful political movements. Cambodia had a built-in constituency for a movement that lionized an agrarian way of life. On the other hand, they had zero hunter-gatherers
and very few idealistic college students, so the chances of an anarcho-primitivist movement getting anywhere at all (and staying there for more than two seconds) is at least approaching ASB territory, if not fully in it.
As a comparison, if I proposed an ATL Hitler who tried to exterminate Calvinists, and someone replied that that's pretty far-fetched, I don't think I could just rejoinder that with "Well, Nazism itself is pretty far-fetched, look at the holocaust". Because a maniac raised in anti-semitic Europe in the era of Social Darwinism could plausibly come up with an ideology combining those ideas in a genocidal fashion. Not so much C20 anti-Calvinism, even if that had historically been a thing in Catholic circles.
Good analogy. I think this site
would write off ATL anti-Calvinist Hitler, for the reasons you point out. But even that is nowhere near as wild as Khmer Rouge-enforced hunting and gathering. Just look at the repercussions for people. The Calvinists are screwed, obviously. Everybody else has to deal with the Calvinists all being gone. Aside from those admittedly terrible things though, life is pretty much the same. Removing Calvinists from the equation is not some all-encompassing change to German life.
Removing agriculture is another thing entirely. Every single person in Cambodia is seeing their way of life change down to the most basic level. OTL you had urbanites accustomed to buying rice being forced to harvest it along with the millions of Cambodians who were already doing that. In the proposed ATL, rice is now forbidden. All the staple foods are off-limits, as are the materials people make their clothes out of. Fish are still fine, but aside from that basically nobody knows what to eat, and there are no preexisting foragers in the region for guidance.
Industrialization? Deindustrialization? Collectivization? Transitioning between feudalism, capitalism, and/or communism? All practically minor tweaks compared with a switch to subsistence foraging in a region that's had intensive agriculture for millennia. Alt-Pol Pot would be removed from office before he had a chance to figure out how to implement such a drastic change.