Many southern styles look like Muay Boran. Old Muay Thai has a lot in common with what we call "traditional" martial arts.
I've never heard any convincing evidence from any view point. But based on the appearance, the forms in Muay Boran, the fact that they used poems for naming techniques and other circumstantial evidence, I believe there was probably a Chinese connection. Of course there may not have been as well.
I would be more inclined to consider Burma than Cambodia for origins of Muay Thai.