I'm not certain but I think most of that open space is less seismically stable than the areas of the island that are planned for building. I worked on the island in 1989 and recall sand boils as well as severe building damage in those areas. That's probably why they aren't planning to build there and are even turning part of into a lake (maybe they figure nature would do that eventually anyway). As to the "farm", maybe it's one of those coop things where individula residents can get a small plot and grow their own veggies. Those have existed elsewhere in the city.