Some issues raised by Chomsky most certainly can be verified by third-parties: many of his recent articles involve East Timor. Did Indonesia commit genocide against them? Did we support Indonesia during this period?
Not having lived in East Timor, I don't know the answers here. But this issue seems far more tangible than the nebulous issues raised by Horowitz against Chomsky (regarding U.S. intentions post-WW2).
Should the answer be yes to the above questions, it by no means demonstrates the infallibility of Chomsky. It demonstrates that in regard to this particular issue he is accurate.
The attempt to dismiss a vast body of work based on several of its positions is absurd. It is more than feasible that Chomsky suffers from a malady common to most humans: he is correct about some issues, wrong about others.
The tendency to overstate one's case is an indulgence few can resist. Both Chomsky and Horowitz rarely deny themselves this gratification.