You're right, it's a fight. But Ronda is the more complete fighter. Sure her boxing isn't anywhere near Mayweather's, but how often does he train kicks? Knees? What about checking leg kicks? What about his ability to avoid combat strikes? You can't just stand in front of someone and trade and slip. What about the combos? Sure her boxing is relatively horrible, but who says she's counting on her punches landing? What if she throws a 1 2 to set up a roundhouse, which is to set up a level change or a tie; and even if she's not successful on the first attempt, it's only a matter of time. There's way too much happening for him to know how to, not only defend, but also stay on the offensive, properly.
Final thought: all other things being equal, if I was a champion, and I got to pick my next fight, and my choices were a world champion grappler, a world champion kick boxer, and a world champion boxer, I'd pick the boxer.
There's always the chance it could happen. Pretty sure I read something about a mixed gender mma fight a while back.