Technically they forbid inebriation I think, but whats the point of alcohol without that?
Nope, both strictly forbid alcohol, period. The Islamic prohibition is slightly more open to interpretation, but the majority of scholars interpret it to mean "absolutely none". The Mormon prohibition is pretty clear though.
Or is it that a Mormon and a Muslim are both, stereotypical, unlikely to be found in a bar, especially in a casual chat with each other?
Not just stereotypically - both religions explicitly forbid alcohol 
While true, it doesn't necessarily preclude them from having a conversation at a bar, though it would likely require strange circumstances.
This is true; it would have worked if the joke had run "sitting in a bar, having a beer" though
