Most messaging apps are centralized. Some form of server side checking can be easily added to make sure the client is legit and not tampered with. Same form of checking thing if it's P2P but it would be very cumbersome.

Now it is possible that someones has cracked the encrypted messaged payload and figured out how to fool the client authenticity checking part but at that point you are probably dealing with a state actor and even apple with their secure app Store cannot save you.