> We do not pretend to know where the truth lies amidst these sharply conflicting assessments; rather, we again want to emphasize some crucial points. 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.[14]
Having read this and what he wrote about this in "Manufacturing Consent", my interpretation is not so much him downplaying the genocide itself, but calling out US media for downplaying genocides and atrocities committed by US-friendly regimes and emphasizing ones done by non-friendly regimes.
So his whole point is that a supposedly "unbiased" US media in general serves to bolster the "Market Ideology" (his words) that dominates in the US and dehumanize its enemies.