The point is that whatever you would need to make Mars habitable could _directly apply_ to keeping Earth habitable at orders of magnitude less difficulty.

Imagine trying to build up population centers at the bottom of the ocean in order to have a backup plan for some nuclear winter on the surface. You could just build your hermetically-sealed, closed-system things on the ground! They'd work just as well (absent, I guess, the getting nuked part). Mars is that, plus you gotta actually get over there to send resources for the first couple hundred years.