I think you are arguing a moot point. It's not really about what happened, the law is mainly about the intent. He did not immediately leave because he did not believe this to be a genuine group. So he has no intend to do harm at this point. But once he becomes convinced this is real he now _intentionally_ receives classified information he's not authorised to have. So now it is _necessarily_ also a legal problem for him.

So in your case: Getting added to a random signal chat where you are not exposed to anything? You should be fine. At least it will be very hard to show any intent to violate anything. Though that isn't necessarily true either. E.g. one could imagine Signal usernames belonging to operatives being in that group. And starting to post or investigate those could still get you in legitimate trouble.

And yes, ignorance of the law does not necessarily save one either. But a central point in all prosecutions is some portion of criminal intent

No, the crime isn't staying in. The crime (if it is one) is choosing to stay in. If he reports on messages from 6mo ago with an auto-delete of 1wk, he was aware of it roughly 6mo ago (and taking it seriously enough to preserve things).