It's hard to figure out unless they had persistent memory elsewhere.
The reason they could tell that a message was deleted is because the existence of a "message deleted" message is itself a message telling them they are be moderated. The lack of this message may make it impossible to determine if this is the first attempt at comms or the 1000th.
> After this, they begin to make backup pages whose names start with “ZZZ” so they will last longer before deletion.
Missed an opportunity here to gaslight them: restore the site from backup every eight hours.
I truly do wonder how that would have turned out. Would they have figured out what was going on?