My gut, too, is that it's seems a bit too tricky. I suppose I should have put 'forest' in quotes originally, because I don't think it's meant to be a literal forest. But it is a neat way (in theory, at this point) to help mitigate pollution and create habitat for birds and insects in an otherwise inhospitable place. I think the idea behind the idea, that cites need to be designed in a way that allows nature in, is a good one.
It's actually, I believe, a huge hydroponic system. The trees have been specially grown to be able to survive like that, and I would guess that's how they avoid the tree roots ripping apart the buildings as they grow.
Of course, a better way to invite nature into the urban environment is to design cities better to begin with. But maybe more difficult with a city already developed, so this is one idea of a retrofit. I wonder if other plant life might not work better, like big hanging plants that they could then "chop and drop", removing plant matter (and thus sequestered carbon and pollution) to maybe a big
compost pile on the edge of the city. Could be collected along with food scraps, to return nutrients to the place where the food for the city was being produced.