A lot of history books claim that Buddhism came to Cambodia in the 14th century and it was the Siem/Thai that introduce Buddhism to Cambodia.
well there's a book call Asiatic Mythology(1932), is about a guy name M. Adhemard Leclere who travel all over Asia and read almost every Asian culture ancient text, including ancient Khmer text books.
Here what he have to say about when Buddhism arrival in Cambodia.
then he stated that Cambodian legend claim that Angkor Wat was built at the order of Preah En to serve as a palace for his son young prince Ket Mealea and to keep the Buddhism religion alive in Cambodia.
if this is true then it mean when it come to Khmer people, Hindu & Buddhism is really the same religion. This also might explain why Angkor Wat can also be a Buddhist temple even tho it have Hindu god and Hindu epic all over it wall.
and in his book he also mention a ancient Khmer book call the Kampi Preah Thomma Chhean, it talk about orgin of Human. What shock me was, it was kind of similar to the Bible origin of Human.