That source doesn't say anything that adds credibility to your point. It's just waffling about the US's involvement in the civil war. By contrast, the WikiLeaks dump of the 1973 diplomatic cables undermine what you're saying.

It's no longer a question of whether or not the US supported the Khmer Rouge. The question now is how much did the US support them. Some members of the state department at the time suggested that the US orchestrated the whole thing. While that seems unlikely to me, it's worth wondering about.