I think this is a decent analogy in concept. It does get to the idea that competition training requires some things that defense training doesn't (higher speeds, sneaky passing, etc.), and vice-versa (intersections, speed limits, red lights, etc.).
The analogy does miss in some areas. If someone were only trained for track racing, they'd completely suck on the streets (no understanding of red lights, can't park, speeding everywhere, etc.). We wouldn't expect deficiencies of that magnitude from competition training (assuming it's hard training like MMA). And race driving is inherently more dangerous than driving on the street. There are fewer dangers numerically (fewer cars, no intersections, etc), but the magnitude of risk is so much higher. That's probably reversed from the competition vs. defense side, where there are more frequent dangers in competition (every moment you're in an MMA match, someone's trying to hit you, etc.) and more significant dangers on the street (someone might try to kill you, which they probably won't in the ring).