This is true. There is no overlap in performance. You can do a bunch of mods to a mountain bike such as installing high-pressure slicks, ditching the useless front suspension, lowering the bar position etc. But a road bike will always be faster.

The other negative of MTBs on the road are the flat bars. After 10 miles with no change in hand position possible, your arms and hands will be asleep.

I don't know why so many people buy mountain bikes. I would guess that only one in 10 ever see real mountain biking, with most never even venturing off-pavement. It must be some kind of weird customer preference thing, where the vast majority of the cycling public would be better served by a hybrid, but they just don't sell.

As far as getting a better workout on a MTB, this is untrue. You will expend the same effort on the road bike, but you will travel farther and/or faster. On an MTB you will be more likely to give up riding, due to the impediments extra weight and sluggish wheels/tires. And you will be more frustrated and more self-critical in seeing legions of other riders pass you like you are standing still.