That's a legitimate question, the soviet's did ''win'' kind of actually in OTL, in three years once they started removing the population their was not a single soul left alive or a village standing near the roads to ambush companies and Khad did use chemicals weapons in the water and likely bio (things like diseases) against their own people.
Pol Pot was disturbingly less a messiah/cult leader and group was more or less nexus for centuries of psychotic hatred and rage against the world for Cambodia.
It must be remembered that at 1979 the Khmer Rouge army was a different beast to the pre 1975 army the American faced. The pre-75 army did not compose of the Khmer Rouge alone but was a mixture of force: Khmer Rouge, Sihanouk's FUNK, the Khmer Rumdo. They were also heavily backed by foreign force with the PAVN sending troops to fight alongside them (called sector Đ, later K for Kampuchea) and Soviet, Czech, China supplying them with heavy equipment. Therefore they were a potent force.
The Khmer Rouge army of 1979 was a hollow shell of that army. First, it enjoyed no support with only Beijing backing it (and even then Beijing was in shambles). Secondly, all of its experts were dead as it went through a purge worse than Stalin's, depleting its army of not only experts but also men with experience.
On the other hand you had Vietnam, the little Prussia of Southeast Asia, with one of the world's largest standing army, armed with latest Soviet tech, honed in the recent bloody war with the US, supported by the Warsaw pact. No surprised the Khmer Rouge lost in a week.
But then, they managed to turn the damn war into Vietnam's very own Vietnam. There is a lack of literature on this, seeing both sides were not the book-worm types nor were they the type to respect their opponents and accept their flaws, but suffice to say the war went badly for Vietnam. The "official" number said 15,000 Vietnamese troops were dead, but I highly doubt that. For example in the book "The Winning side", Huy Đức, who had access to state archive, said 100,000 Vietnamese troops were killed during the 10 years in Cambodia.
So yes, a tattered army, a bunch of angry teenagers managing to stop and bog down the toughest, most experienced, best-armed army in Southeast Asia at that time, an army that was expected to go toe-to-toe with China to protect the Soviet union is an army not to be underestimated. The only reason why Vietnam won that war, a.k.a by setting up a regime that is kind of pro-Vietnam and making sure that regime is safe from its enemy, is because Khmer Rouge lost its backers. By the end of 1991 Vietnam was beyond exhausted and if China decided to continue funding the Khmer Rouge, I would suspect the Khmer Rouge would take control in the same manner as the Taliban in Afghanistan
As long as China, US and Thailand can keep funding and apply pressure the war won't end like Hutu and Tulsi conflict is still going on from 1959-till nowadays arguably. Even Vietnam's rather genocidal campaign failed to stop them as long as they have Thailand.
You have a good point on Egypt, while it should be producing tens of thousands of ''freedom fighters'' once the military high ranked remember they will one come back and how they see Israeli collaborators and demand power and to set up a Islamic state with many of likely being killed for their actions against them ( qutb spent years being tortured, a action many still don't forget today) they certainly will keep their options open for offers.
Though I think the West would overlook Taiwan as it's their partners in Asia who bear the highest risk and the current strategy damaged the USSR, but failed to break it so sunk cost.
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