I think it describes the suburbs that a lot of these people are from. I don't think it characterizes North American cities, which this article definitely thinks it does. I was struggling to find anything distinct between Montreal and Chicago from all of these pictures. And I've lived in a lot of different large, small, and medium-sized US cities in my twisty journey from Chicago to Chicago, although my only coastal cities are Portland, OR and Baltimore.

I think the Toronto distribution (which I'm willing to assume is accurately described) is probably characteristic of a particular very rapid growth pattern with a particular income distribution, rather than some North American collective mistake.