It's not a non-sequitur - people are calling for death [1] (somehow this doesn't get a mention in the NPR article - an honest oversight, I'm sure!), while the post claimed they were being fired for constitutionally protected speech. So it makes sense to ask if calls for death are protected.

To answer that question, I think it's close? It's not imminent lawless action or a true threat, which are the usual criteria for when the 1st Amendment doesn't shield speech. But it could be interpreted as instructing someone to commit a crime. But since it could be argued the posters have no reasonable expectation their instructions will be carried out [2], that means it's probably (speculating, I don't know of any precedent on this) protected. In the USA - most other countries are not nearly so permissive.

[1] https://thatparkplace.com/bluesky-users-call-for-death-of-do...

[2] Carried out specifically because they issued them, and not because someone was going to kill Rowling anyway.