IIUC, it becomes discerned by US law if someone acts on violent rhetoric. Then, the violent rhetoric can be used as evidence that the actions were part of a hate crime.
If your speech inspires someone to shoot someone an hour from now, it's legal. It wasn't imminent. If you speech will inspires someone to commit a non-violent but illegal action immediately, such as buying some drugs, it's potentially illegal (outside of 1st amendment protection).
Unless (a) directly incited (b) (via the boundary test you cited), (a) isn't legally guilty of anything.
But if the law discovers (b) heard (a) and acted because they believed (a)'s rhetoric, (b)'s crime rises past just violent crime to hate crime (because their reason to commit violence was a hate-crime motivation).