Heartfield has never said that mass killings did not take place in 1994. (The history of Rwanda did not begin and end in 1994, however; to explain the killings, you need to understand what went on before and after them.)
More to the point, your claim that the USA failed to intervene is contrary to the truth. Just because the US government did not bomb Rwanda does not mean that it never intervened. It intervened by supporting Paul Kagame.
In this particular US intervention, the stakes for the USA are, ultimately, probably not Rwanda but Congo (formerly Zaire). The US government finally decided that it wanted Mobutu out (dictatorships of the old-fashioned kind ceased to be a useful defense against Communism, now that the threat of Communism disappeared from the world stage) & sought to replace him by a new instrument. Kabila went out of American favor after his break with the Rwandans, but the USA can still use Kagame to help it reshape the Central African region.
Yoshie