I don't think anyone is seriously arguing that Mars will be more habitable than Earth. The argument is about the possibility of humans on Earth being wiped out due to freak events like a huge asteroid impact or global thermonuclear war. Earth would still be more habitable than Mars, but the probability that human survivors would be equipped with Mars-level survival tools is tiny, and any facility equipped like this would have to be hardened against desperate survivors trying to take it over and bringing it over capacity. Meanwhile if we had a self-sufficient Mars colony they could resettle any Earth that is more habitable than Mars.

Now I'm not saying it's necessarily a smart allocation of resources. But it does follow the popular IT saying "one is none, two is one. If you care about something make sure you have a backup"

Yeah a future where Mars is more inhabitable than earth is unbelievably depressing.