Inciting violence can be protected by the First Ammendment if it isn't inciting imminent illegal action. His post is probably protected. If he were pointing at a guy and saying "We should lynch that ICE agent right now!", that wouldn't be protected speech.

FWIW, I don't even remotely agree with what he's saying.

This is true but HN does not claim to be a free speech site and instead is focused on interesting discussion. There are many things that are legally protected speech that will get you reprimanded or outright banned here.

"Shouldn't be allowed" is ambiguous.

To add to this, the speech needs to be imminent, specific and actionable.

For example, "People should [bad thing] ICE agents" would be protected speech as it isn't imminent, specific or actionable.