Seems an overreaching and incorrect statement. Lots of studies on the correlation of outdoor time and well-being. This study itself, though imperfect controls for some of those variables like exercise.

It might not be true for everybody (sounds like you're fine sitting inside, popping vitamins, and grinding on the Peloton or whatever -- good for you) but don't make the mistake of thinking your experiences are others'.

Clearly there are things about "the outdoors" that can't be replicated indoors. Smells, the sheer size and variety of environs, O2/CO2 levels, etc. Unless you are living in some kind of enormous, elaborate bio-dome.