This is a situation in which I am happy to defend Chomsky. At the point of writing there was little evidence for some of the most extraordinary claims, claims which were similarly leveled from propaganda channels against socialist regimes in Vietnam and else were - often without a base. Chomsky did alter his opinion based on new sources and contrasting evidence. However the people who do critique Chomsky do it mostly for anti communist reasons, ignore his later writings and at the same time ignore that the USA did support the Red Khmer after the evidence came out for quite a bit.

Let me quote wikipedia to make a point about how it is leveled and against whom it is leveled:

In addition to Chomsky, Porter, and Hildebrand, the atrocities of the Khmer Rouge have also been denied and/or whitewashed by such academics as Marxist scholar Malcolm Caldwell, Laura Summers,[18] Edward S. Herman, and Torben Retbøll.[19]