You are probably recycling the CO2 from the astronauts breath which is easier than cracking it from the air. Also many practical breathing systems (especially for spacesuits) leak some of the inert gas for cooling and other purposes.
The real thing you’ll need O2 for is an oxidizer for fuel but of course you need to make fuel too.
If you are interested in making anything interesting such as large plastic sails or small biospheres nitrogen is the big missing piece of the puzzle right now in the moon, mars and asteroids.
CO2 needs to be scrubbed/vented out of the breathable atmosphere. Even if there is abundant of oxygen anyway, too much CO2 will make people feel dizzy and confused, cause splitting headaches, increased heart rated, reduced senses, and eventually death. CO2 can't take the place of inert nitrogen in a breathable atmosphere, that much CO2 in the air just isn't compatible with human life.
being extremely generous, and assuming he meant "martian atmosphere" when he said "air", it _might_ be a bit more challenging (depending on your process and what you've got available) to convert the CO2 at 0.095 PSI into O2, than it would be to pull the CO2 out of ~14.7 PSI human exhalations and convert it to O2.
seems like the kind of thing where there is a bunch of engineering that could be productively done.
The real thing you’ll need O2 for is an oxidizer for fuel but of course you need to make fuel too.
If you are interested in making anything interesting such as large plastic sails or small biospheres nitrogen is the big missing piece of the puzzle right now in the moon, mars and asteroids.