[acdha]

It’s done with at least two external inputs (air, water) and far more resource availability than a Mars mission would have. A long-term closed loop isn’t banned by treaties - McMurdo alone is like 50 acres and a hundred buildings, something the size of a plausible interplanetary mission at our current technology level is not going to dramatically exceed that footprint.

Again, I’m not saying it’s inconceivable that it could be done, only that it’s harder than the sales guy would have you believe.

[somenameforme]

Mars has more than sufficient resources to provide practically endless air and water as well.

Beyond that I think you're also on a red herring here. There's no plan for a long-term closed loop on Mars to begin with. In the distant future most likely, but complete self sustainability is not practical in short to mid term timeframes. That would require essentially duplicating absolutely all forms of industry on Mars which probably will happen but only in the very distant future. In the interim a Mars colony would be receiving regularly shipments from Earth, and those return trips would also enable colonists, who decide it's not for them, to also return to Earth.