So, if you restrict yourself to downtown Seattle, it looks like a clear divide, but if you go to the Seattle city limits, it doesn't, and if you go to the Seattle metro area, it looks even less like that?
What's the problem supposed to be? And why is "North Seattle" being used to refer to the northern half of downtown, rather than the northern part of Seattle?
This is exactly why local/micro-level expertise is so important. It's common in the US now for areas just on the city limits of bigger cities to be more mixed racially and socioeconomically than ever before; this is a temporal as well as spatial phenomenon driven by the change from "white flight" to the suburbs to instead lots of folks moving to the suburbs because they're cheaper than the downtowns, and the suburbs mixing racially because they've removed their sundown laws and racial covenants. Now many downtown areas are coveted instead of abandoned, with luxury condos instead of skid row.
I don't know anything about Seattle. If you look at the area of the clear divide, what differentiates the houses on one side from the houses on the other side? In Minneapolis-St Paul, there are pretty clear differences in amenities between many racially separated areas: more trees and walking paths in some areas; fewer trees, worse streets, and highway noise in others.
What are you asking when you ask, "What's the problem supposed to be?" I don't quite get it.
Do you see how Southeast, Dunwamish, and Delridge are a rainbow of colors with barely any specks of blue while the north neighborhoods are all blue with specks of green?
What's the problem supposed to be? And why is "North Seattle" being used to refer to the northern half of downtown, rather than the northern part of Seattle?