However, some UA crops (for example, tomatoes) and sites (for example, 25% of individually managed gardens) outperform conventional agriculture. These exceptions suggest that UA practitioners can reduce their climate impacts by cultivating crops that are typically greenhouse-grown or air-freighted, maintaining UA sites for many years, and leveraging circularity (waste as inputs).
Tomatos it is then ;)
It's really hard to compete with the efficiency that economies of scale provide. So this result isn't unexpected.
It doesn't however negate the other positive impacts of urban gardening -- in particular, the impacts on the people doing the gardening (everything from psychology, vitamin D, immune system benefits to playing in the dirt, etc.).