The KPNLF were in a coalition government from 1982 on with Sihanouk and the communists, whom you call the "Khmer Rouge". In fact the coalition with the communists happened as soon as the US began funding them - it was probably ordered by the US.

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.