"Discord’s Counter-Extremism analysts proactively detected, removed, and reported this user to the FBI’s National Threat Operations Center, which is what led to the user’s arrest." (from the liked article)

So I would hazard a guess at keyword matching algorithms. No need for a LLM. But automated most likey.

i recall a little while ago, voices raised about discord being used for things it wasnt made for. Community watch seems to be in play