I always assumed the warrant canary works because no law can force you to post something online, and even if they could, it's going to take longer than the time it takes your canary to expire.
That's why the canary should also say that you haven't been contacted by law enforcement for subpoenas, requests, warrants, etc.
It won't protect from a dishonest admin (but you should not be using such a site in the first place if that is the case), and the admin cannot be
forced to update the canary and keep the "I haven't been contacted by law enforcement" part intact.