When the US begins arming the coalition of Sihanouk and the communists in 1979, then all the arms are going to "rival groups" in the Sihanouk/communist coalition. It's ridiculous.
Any how, the US fairly openly armed what you call the Khmer Rouge, I guess a sliver of plausible deniability is enough for the minority of people like you who want to deny it. Nightline even did a whole segment on it.
The KPNLF were essentially strong armed into a coalition government by the Thai - who threatened to cut them off from aid and boot them from the country - and to gain international recognition and aid. They waited until the last possible moment to join and remained hostile towards the Khmer Rouge.
If you have a link to this Nightline episode, I'd love to see it. I couldn't find it.