Disingenuous or hopelessly ignorant? I'm not sure.

They mention Trump, for reasons, and try to paint this 'cancel culture' as a problem for both the extremes of both left and right. Have they been asleep for a decade? The cancelling is overwhelmingly coming from the left. Whether it's maniacs on Twitter, Californian tech/media companies, the corporate world, or academia. Nobody is getting cancelled for saying that men are shit or that blacks and women are systemically oppressed. Nobody is getting cancelled for suggesting that children should be sexualised. When Rowling began her bizarre virtue signalling nobody was calling on her publisher or her agent to drop her. Rushdie and other critics of Islam are not being hounded by the right. Chomsky doesn't have right-wing Twitter mobs denouncing him. You're not going to see your career in tech, video games, films, TV, or comics ended for arguing for left-wing identitarian positions, but opposition will certainly be risky.

It should surely have been a hint that at least one signatory pulled out of the letter because they didn't want to be associated with some of the people who had signed the letter - a letter asking for an end to cancel-culture. The letter strikes me as big yet more virtue signalling by many whose views on the mob changed only after turning around to realise the mob wasn't running with them, rather it was chasing them.