> How is Mercury further?

Δv, the only metric that matters. Mercury is at 5.5 km/s from LEO, vs 3.6 km/s from LEO for Mars.

To be fair to JumpCrisscross, he never claimed Mercury was further away. It's just "further".

> Mars's atmosphere is pretty useless for humans

Oxygen, carbon, nitrogen and hydrogen make up 95+ percent of a human by mass. Mars has all four, the first three in the atmosphere.