Nice try

https://en.wikipedia.org/wiki/Cambodian_genocide_denial#Chom...

I do not read anything, close of a statement "cambodian genocide is just us propaganda"

What I read is:

"What filters through to the American public is a seriously distorted version of the evidence available, emphasizing alleged Khmer Rouge atrocities and downplaying or ignoring the crucial U.S. role, direct and indirect, in the torment that Cambodia has suffered"

a much more nuanced statement.

Are we reading the same quote? He is calling reporting about the genocide propaganda.

I am reading, he is calling some reporting possible propaganda.

the best propaganda is the correct truth, no?

Correct, but Chomsky wanted people to believe it was lies.

Because he believed so, too. The US would have had incentive to paint the picture worse of the communist side doings after they left the area in a mess. After the pentagon papers and everything, that would have been a plausible scenario - it just happened to be real murder on big scale, even unseen in sowjet russia before.