It all sounds pretty abstract, I know, but it's actually quite comprehensible and relevant, as a couple of examples might clarify. Someone might begin to believe the JFK hypothesis from intuition, a guess, watching Stone's movie and finding it plausible, reading Lane's book and finding it plausible, investigating documents, etc. For Chomsky, it may even be hard to conceive how a serious longtime student of social relations could believe the JFK claim. One option is to just ignore the claim or dismiss it out of hand. This has the virtue of being quick and trouble free and of seeming proportionate to the worth of the hypothesis.
Many people begin to take it quite seriously.
The next option for someone in Chomsky's position is to briefly summarize a theory of society and point out that the hypothesis is inconsistent with that repeatedly verified theory and therefore must be wrong. This is okay, at the beginning of the exchange, but at a late stage it would be sectarian. It asks the adherents of the now quite widely held hypothesis to drop it merely because it contradicts a theory someone else, or even they, previously believed. We are open to new insights. Well, if you take a look at Chomsky's article in this issue, that's what it does.