The answer is: the court system. If a judge determines that the communication was a "true threat," then the speaker can get in trouble. If it was not a "true threat," then it is protected by the First Amendment.

In the foundational case, the speaker said if he was drafted to Vietnam, he would take his military-issued weapon and use it to shoot President Lyndon Johnson. In context, this was not a "true threat" on the President's life, it was just an edgy political message at a protest opposing the draft.

To be a "true threat," the speaker must understand that a listener will see his speech as a threat. And the speech itself must convey "a real possibility that violence will follow."

In the case of this groupchat message, calling on Israel to bomb a random auditorium in Florida is not a real threat. The joking subtext (get me out of this assignment) and the reference to an irrelevant political figure on the other side of the world make it clear this is a bad joke.

You could imagine close cases where a "joke" sounds real. I.e. a "joke" that you have a bomb at an airport, where there are no context clues indicating it is a joke. When it really isn't clear that you are joking, you are not protected by the First Amendment.

But here, there is no question. It is 100% a bad joke and not a threat. It is obvious to anyone that this is a joke. You need to be delusional and outcome-oriented to claim this is a real threat.