An odd book. There's a germ of an idea at the core, about his wariness of urban planners/architects/public policy people wanting to push "more trees" as a solution to all of the ills of society as being, at best, naive, and at worst, an active attempt to divert from the actual work of helping people and tackling structural inequality and poverty, but it gets very blurred. He makes some not entirely convincing links between some of the proto-fascist and eugenicist thinking of the early advocates for garden cities and other types of planned urban spaces and contemporary efforts to "re-wild" cities where really it just seems like he finds the latter a bit silly and wanted the chance to rant at the hippies who creep him out. I don't think this is any clearer than when he frequently simply describes things, people and ideas as "weird" without any real reflection; it's a collection of impressions and suspicions, which fine, but I don't think there's any grand thesis here worth that much of anyone's time or really worth dedicating a whole book toward.
If I had to draw any real conclusion it seems to be that those involved in city management should stop attaching grand ideological narratives aimed at shaping the "character"/"moral fibre" of their residents or "returning" city dwellers to some anachronistic idea of "nature", when making plans for their cities and instead focus on making interventions which tangibly improve people's lives in simple, practical, ways, but he doesn't even spell this out, that's just my inference based on the extending grumbling throughout the book.