Signal and other OTR-derived systems actually cannot cryptographically prove that someone sent a message (the recipient can forge the message because the message is authenticated via HMAC not a signature).
To be honest, I'm not convinced that this is practically true in Matrix except in cases where you wish to argue that the homeserver operator is "in on it" to create fraudulent messages (the same goes for Signal -- though in Matrix's case the homeserver usually keeps the entire conversation history, making it even harder to make that argument). So while it is technically true that you cannot cryptographically prove that a message was sent by an individual, practically speaking you probably can prove it.
