I think the benefit of rooftop gardens is taking what is essentially "wasted" space (where I live most rooftops go completely unused) and repurposing it into something productive.
Having attempted growing vegetables on the rooftop of a 6 story building, I discovered that preparing and cleaning the roof is incredibly energy intensive. You have to bring soil from some soil farm to the city and then onto the roof of a building where you have special beds setup and a way for runoff to be cleaned and processed. Water, nutrients, tools, everything else has to be brought up there. Dust blown off the beds when it gets a bit dry flies all over the city. It also gets too hot up there because the roofs are super hot, so you need more water. Then the birds, squirrels who are actually able to climb as high as necessary, neighbors who live below the roof, and other problems creep up that were totally unexpected. Good luck to many others who will try the same and find out this on their own.
It sounds like they're using aeroponics where possible. Of course that still requires energy, but much less so than growing in soil. There's also much less overall water use, because it's essentially a closed loop system with minimal evaporation.