I don't think that needs to be called out any time Chomsky says anything; it's usually not relevant to what he's talking about. But in this particular instance, it seems extremely relevant.
Of course, plenty of Chomsky’s opponents have their own rhetorical and moral problems. As an undergraduate I watched Chomsky debate Alan Dershowitz, and Dershowitz put on one of the most disrespectful (of Chomsky and the audience) and academically dishonest performances I have ever seen.
It is unfortunate that many outspoken US detractors lose their voices once Russia or others commit clear atrocities, or just keep repeating non sequiturs about the US. It makes it harder to trust their earlier arguments and disentangle principle from reflexive anti-Americanism.
(It is likewise very damaging when the American president/military/intelligence undertakes aggressive wars, arms/trains paramilitaries in authoritarian countries, supplies weapons to murderous dictators, etc., first because those are bad in themselves, but also because they undermine American international legitimacy in cases where projection of American power can actually defend freedom and peace.)
It is no wonder he's so depressed about the future, the belief that anyone with power is allied with everyone else with power on every conceivable issue is as good as forecasting permanent defeat. For example he writes off the end of the Vietnam war as something that happened when business interests turned against it, not really thinking about the fact that business interests are not completely inaccessible to normal people and in fact are normal people. They're just not academics.
I think this is an unfair criticism for a couple of reasons:
1. He rolled back his factual claims when more evidence came in. I don't think you can call him a denier in the present tense.
2. In a sense it is Chomsky's "job" to question everything the US media is saying and of course this will sometimes involve criticizing cases made that are actually right. (To say otherwise would be to suggest that the US media is always wrong, an extreme position for anyone.) Furthermore Chomsky can still be right in a sideways way when he criticizes true reports if the media does a hack job of reporting it, kind of like how criminals can rightfully get off on a technicality if the prosecution is incompetent.
His present views on the issue capture both of these points, although it would have been nice if he expressly said "oops, sorry:"
> As we also noted from the first paragraph of our earlier review of this material, to which we will simply refer here for specifics, “there is no difficulty in documenting major atrocities and oppression, primarily from the reports of refugees”; there is little doubt that “the record of atrocities in Cambodia is substantial and often gruesome” and represents “a fearful toll”; “when the facts are in, it may turn out that the more extreme condemnations were in fact correct,” although if so, “it will in no way alter the conclusions we have reached on the central question addressed here: how the available facts were selected, modified, or sometimes invented to create a certain image offered to the general population. The answer to this question seems clear, and it is unaffected by whatever may yet be discovered about Cambodia in the future.”
https://en.wikipedia.org/wiki/Cambodian_genocide_denial