1979 was after the genocide and after Pol Pot was pushed out of power. Implying the US had something to do with the killing fields defies common sense. The khmer rouges were primarily China and North Vietnam backed.

Now the US did support some incompetent and corrupt militia in Cambodia to oppose the Khmer rouges, and those did their fair share of misdeeds, to the frustration of local US officers. But given the crimes the khmer rouges ended up committing, it is hard to argue that not opposing them was the morally superior position, even with hindsight.