Perhaps reality is not on the extreme ends of the spectrum.
Maybe cycling is good exercise, but it should be supplemented with other forms of exercise if you want a full body workout.
...and to be clear, mountain biking can use a lot more than just your legs. There can be a lot of core work thru balancing and throwing the bike around singletrack. Core, arms, legs- thats about everything.
But yeah, to have overall balanced strength and fitness, you should target more than just one area.
whoda thunk?