Nowadays Chomsky points out something you can read in the December 23rd, 1985 edition of the New York Times - that six years after the hue and cry all over the US about "Cambodian genocide", the US began sending millions to Cambodia in order to arm Pol Pot.
Of course at this time the US was using its power at the UN to keep the Cambodian UN seat in the hands of the political coalition that the Cambodian communists belonged to.
He also points out the mass murder the US air force carried out in Cambodia, bombing rural peasant villages. That the US media ignored a genocide in East Timor that was happening at the time of the communists taking power in Cambodia.
Insofar as "denial of the Cambodian genocide" by Chomsky - could you give an example of a statement he made about Cambodia which was not true? If you are unable to do that, it is just mud-throwing - in some other thread he's said to be denying Nazi genocide in Europe because he was opposed to the jailing of a French professor accused of being a holocaust denier.