Definitely this and for this reason. Hard to add someone without clearance if they aren’t even on the list.
Now, the interesting question is: did someone add that fellow to the chat for a reason? Some disgruntled- or fired - aid wanted to do a huge FU? Again, if they were using the appropriate communications channel, this never would have happened.
If it’s an aid, I’d happily buy them lunch. That person is a hero.