The point about "feeding martians" is about the sustainability of a colony; it would be very expensive to transport food from here.

This is not a discussion of "open air" farming. Water used once can be recycled and used again; water taken up by plants and emitted can also be captured and reused. Further, it has been shown that Mars does have water.

Actually the downside to this seems to me to be one that was not mentioned; under the low gravity conditions, if you tracked a bit of mud into the habitat it woulkd become an excellent bacterial breeding ground.