I don't think that's true, except for the server? From the client's perspective, the current state is whatever the hell the server thinks is the sum of all previous state. So you generally don't need lossless message passing; you just need to be able to resync periodically, and trying to resend on lost messages is probably a waste of time if you can sync straight to current state

If the client/server were deterministically simulated -- thus every event must be fully represented on both sides to be in sync -- then sure, but I'm fairly positive no MMO does that