Free speech does not include shouting "fire" in a crowded theater or the like. I thought everybody knew this.
Oliver Wendell Holmes, Jr. regretted ever saying this. It also isn't currently the law anyway. The law as it currently is is stated in
Brandenburg v. Ohio, not in Holmes's unfortunately worded fire in a crowded theater dicta:
"The constitutional guarantees of free speech and free press do not permit a State to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action."
I'll note that applies to the federal government now as well under the doctrine of reverse incorporation, which applies the Constitutional protections against State violations against the federal government as well. Unless speech is both directed toward and also actually likely to result in imminent lawless action, it is protected. Obviously, of course, one could shout fire in a crowded theater if it were actually on fire.
I should note SIG's dumb shitpost not only carried (at least on its face) an incitement to violence, but was itself actually a thread. That is governed by
Watts instead. In this case, the defendant had made this statement: "They always holler at us to get an education. And now I have already received my draft classification as 1-A and I have got to report for my physical this Monday coming. I am not going. If they ever make me carry a rifle the first man I want to get in my sights is L. B. J." This was found not to be a "true threat" but merely political hyperbole. Americans have a long history of saying violent shit and we generally like these rights.
Now, the traits of SIG's post, unfortunately for him, actually meet the criteria, at least if taken as explicit statements of fact rather than what it obviously was to Kiwis which was a shitpost. You can see why feds would at least be interested in ascertaining exactly what the fuck he meant by that statement, and it's possible they'll prosecute him just for wasting their time. I don't think his glowpost warrants a prosecution but the feds may disagree.