I can't resist being pedantic here. Nodevember's shitty tweet does not likely fall under "libel" because it's not a falsehood. It
may be considered defamatory (and libel is a kind of defamation), in the way that "We've decided not to invite Douglas Crockford because we want to maintain an environment where children aren't abused" can be technically true but defamatory by insinuation.
I justify being pedantic here, though, because the justice system is not just a binary system of "Someone got hurt, and now someone will pay in a court of law". Other common-sense factors are taken into consideration (such as the magnitude of alleged defamation, and whether the defendant is a public figure). And in terms of how things are regardless of U.S. civil law, the reality is that Douglas Crockford is Douglas Crockford. Just because some relative nobodies said hurtful things to him and he got disinvited from a privately-run conference doesn't mean he needs to turn to the justice system to find recourse. As we speak, whoever is running Nodevember's Twitter account is probably wishing they were running @CincinnatiZoo, post-Harambe. And the people who made allegations against Crockford are likely getting death threats and other things hurled their way. I'm not implying that I, or Crockford, think that this is the way things should be, but unless Crockford can conclusively argue that being shunned by Nodevember has destroyed his livelihood, arguing that Crockford deserves legal recourse against personally harmful speech would require some radical revamping of First Amendment rights.