[truncheon]

The premise here, being that powerful (or technically adept) people may have the capacity to impersonate a phone number, or clone a SIM card.

...since that part isn't directly stated, and might not be obvious to everyone.

[FabHK]

That would indeed be enough. The other conceivable scenario is that the server gets compromised (and suppresses the "delivery receipts", then sends a spurious "rekeying" message).