I don't think the situations are very similar.
In Dai Viet, Confucianism was endorsed by the ruling dynasty, and its institutions (such as the exam system which made Confucian study a mandatory part of qualification for civil bureaucracy) and places of worship were constructed and patronised by the elite. The population remained overwhelmingly Mahayana Buddhist. The event represents this, as well as a compromise between having no Mahayana nations in 1444 and an accurate depiction of Dai Viet's state religion.
In 'dark age' Cambodia, Buddhism had been both the official religion of the elite and the dominant religion of the population (at least the population of the capital and major urban centres) for a very long time. Angkor Wat was converted to a Buddhist temple some time in the late 1100's. As you quoted, Theravada Buddhism specifically won out by the EU period. I don't see any reason why Cambodia would in 1444 or later decide to revert to Hinduism. If a Khmer player wants to do that, it's not at all difficult to use rebels to achieve this.
In Dai Viet, Confucianism was endorsed by the ruling dynasty, and its institutions (such as the exam system which made Confucian study a mandatory part of qualification for civil bureaucracy) and places of worship were constructed and patronised by the elite. The population remained overwhelmingly Mahayana Buddhist. The event represents this, as well as a compromise between having no Mahayana nations in 1444 and an accurate depiction of Dai Viet's state religion.
In 'dark age' Cambodia, Buddhism had been both the official religion of the elite and the dominant religion of the population (at least the population of the capital and major urban centres) for a very long time. Angkor Wat was converted to a Buddhist temple some time in the late 1100's. As you quoted, Theravada Buddhism specifically won out by the EU period. I don't see any reason why Cambodia would in 1444 or later decide to revert to Hinduism. If a Khmer player wants to do that, it's not at all difficult to use rebels to achieve this.