We already know that there are signers whose names were put on the letter without knowing who the other singers are. People literally got duped into signing their name to what they thought was a puff piece, but in reality must have had bad intentions from the start if they had to trick people into signing it different revisions than the published version. There’s little wiggle room for defending the letter, and anyone who makes their living by assigning their name to ideas that got duped should have their reputation re-evaluated - especially if there becomes a trend of bad decisions.
Chomsky using the argument “the letter about public criticism in media needed to be published because people are critical towards it” is frankly meaningless, but it doesn’t mean he is going to be cancelled because people think he did a dumb thing.