There is no concrete evidence the US armed the Khmer Rouge.
At best, there is some evidence that some of the non-lethal aid provided to rival groups might have made it to the Khmer Rouge, but that's a far cry from arming them.
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.