This is an interesting finding. I don’t know what conclusions the article draws, since the Telegraph doesn’t seem to have linked to it. The conclusions the Telegraph draws are pretty dumb.
To talk about whether these gardens are good or bad for the environment, we would need to consider all kinds of additional factors. What alternative use would the land be put to? Does it make sense to amortize the carbon cost over units of food when the community derives value from it in many ways other than caloric?
It should be unremarkable to all but the most confused flower child that industrial agriculture’s economies of scale are capable of producing food at a lower per unit cost (carbon, money, etc). That doesn’t strike me as a good argument for ditching urban gardens.