What tribe in the northeast? They're all rice farmers, the nearest hunter-gatherers live in the interior of the Malay Peninsula.
I didn't write about a tribe of "hunter-gathers", but a "tribe which has a strong reverence of hunting", i.e. a tribe/village where hunting is idealized as the best way of life, despite the majority of the food coming from rice farming.
Ross (1987, ed.) describes the the tribes in Northeastern Cambodia thus: "Most Khmer Loeu live in scattered temporary villages that have only a few hundred inhabitants. These villages usually are governed by a council of local elders or by a village headman. The Khmer Loeu cultivate a wide variety of plants, but the main crop is dry or upland rice grown by the slash-and-burn method. Hunting, fishing, and gathering supplement the cultivated vegetable foods in the Khmer Loeu diet."
Out of the many thousand villages, I honestly don't see it as a stretch to imagine one, where hunting is seen as something everyone should do at least once or twice a week, nor Pol Pot failing in love with exactly this village and its reverence for hunting.
This brings me to the next objection: wouldn't Pol Pot be removed, if he tries to force this on Cambodia. This is very likely, but less so if he does it slowly, i.e. he starts by mandating that Friday is the mandatory hunting day. Then after some years, he adds Tuesday as well. A few more years down the line, we get a crop failure (because of central planning doing something stupid on a massive scale), and Pol Pot declares that the solution is to the impending food crisis is to do even more hunting and gathering, which causes even more shortage, which leds to calls for even more hunting & gathering.
Of course, at some point, someone is going to try and stop the madness, but this depends on the strength of internal repression (i.e. it is not given that a coup will succeed). The very basic argument against a coup is that Pol Pot killed every fourth Cambodian without anyone saying stop and removing him, so why would the same cronies say stop at killing off every second Cambodian?
Let me clarify that I don't want to argue that this is very likely or indeed even likely (and I concede that my last comment could be read as such), but simply that it is not impossible.
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.
To clarify my comment about OTL-Khmer Rouge being far fetched, I specifically meant the fact that they slaughtered a quarter of the population. With a starting point in the 1960s, I do simply not believe that it is a given outcome that 25% of the population has to die, even if crazy communists like Pol Pot take power.
As for the analoge of Hitler and Calvinist, then I would argue that what we are discussing here, is equivalent of a succesful Generalplan Ost-scenario, victorious national socialism in Europe, i.e. where Hitler wins ww2. In such a scenario, some 20-30 years down the line, where are the Calvinists? At best devoted Calvinists are tolerated, at worst for one reason or another they get on the list for deportation and such. In a regime that relies on the concept of an internal enemy (which appears to be a common feature among communist or national socialist states), there doesn't seem to many limits on who - over time - can become that internal enemy.