Absolutely they’re under threat, that’s not in question. So is the Dominion. So is Kerrigan’s Swarm. The entire galaxy is under threat at that moment.
@Bifrost stated that Artanis knew things were bad on Shakuras specifically, which is incorrect at this point in the story. He believes they will be attacked (logically), but Artanis does not know when.
When he does arrive there, he’s extremely surprised to see how “bad” things actually are; no one suspected Amon would get the Warp Gate activated and invade from that direction until it actually happened.
Unless the canonical mission order is revealed, we also don’t know if Amon does this almost immediately or not.
One day, perhaps we’ll learn.