They are different sports. Haven’t we all been out with the supposedly super fit roadies who aren’t experienced mountain bikers only to find them busted and floundering on a moderately challenging ride? I have.
Furthermore I’ve been going out on night rides (mtb) with a few roadies this winter, the first time for them and they are reporting huge improvements in climbing strength when they get back on the road bike, something echoed by their clubmates who are left trailing behind them.
Riding on the road will improve your cardiovascular fitness for spinning a big gear at a high cadence for a long time. This will have a beneficial effect on aspects of mountain biking which require you to do the same. Mountainbiking is a much more dynamic sport which requires a more general type of fitness.
There are plenty of fat roadies out there too, but in general i would say if you take body fat percentage, lung capacity and heart rate as indicators of fitness then the majority of them will conform to what people regard as fit. In the same way as you would marathon runners I suppose. Neither of whom would last very long in the boxing ring though, when subjected to that type of endurance. Horses for courses etc
super fit?