More importantly, it does not really matter much if the area used for the production does not save on space. That's not really the point, although it would be nice. The point is to bring agricultural production closer to urban environments, in order to save on the energy cost related to the shipping of said product, and possibly to bring far fresher products to customers. The article seems to portray that as a pipe dream. Again, these are things that can surely be minimized in order to compete with shipping between rural and urban areas.
With that out of the way, yes, currently urban farms do seem pretty ineffective and useless. That doesn't mean that we shouldn't explore and develop these ideas. Once upon a time cars were pretty inefficient toys of the ultra rich too.