Go ahead and ride my road bike down Fireswamp or Thieves Forest while I'm on my 203mm DH bike and tell me which is more "physically taxing." This argument can be directed whichever way the arguer would like by picking and choosing what makes something "physically taxing" and ignoring other considerations. If you're talking solely about the effort required to get weight up a hill, then fine (but even there, MTB gearing makes really steep routes much easier, and even possible, than road gearing)--weight alone, all other things equal, makes a difference. But there is a lot more to riding than that (and 300w is 300w regardless of the bike or surface anyway--nobody uses kw when talking about riding a bike).

Then again, I have to remind myself that I'm arguing with someone who said "I would literally die of boredom."