Probably not deliberately, but something can always go wrong, which is probably why they added an additional clause that messages older than 30 days should be deleted.

More interesting is what happens if someone gets a new phone. In that case (if I understood correctly) they might ask for the sender to resend the messages with a different key. If they really are forced to add a backdoor then that's where I would fit in a MITM attack, as it is limited in scale and detectable when used excessively.