Chomsky's point is that the N people that need to be involved is large, so even if you think it's unlikely that one co-conspirator would come forward (you are right on that), when N is large then the probability that at least one will come forward, 1-pow(1-p, N), becomes much more likely.

Your other point about people not being believed is a separate point to the above. Has there been people claiming to have been involved in 9/11 on the inside that haven't been believed?