That map is awesome! Shows how the Sheppard line really hasn't helped increase transit ridership in that area. Maybe it should have been an Eglinton line instead!
With regards to suburb/not suburb I guess there are different definitions here. I would say technically anything outside of the downtown core is a suburb. But there are different levels of suburbia. Rosedale, East York, and other areas directly next to downtown are the older, streetcar suburbs which are a reasonable distance from downtown, built in an era before the car was so widely available. Today these are some of the most desirable neighbourhoods because they are close to downtown while having a more suburban feel.
Don Mills is not one of these suburbs. It was built with the car in mind and as such is much closer to a modern suburb. The only reason one feels different in Don Mills is because it is a more mature area - trees are taller and more lush, houses aren't as cookie cutter and have had time to be remodelled and redesigned by a set of subsequent owners, and homes have a different design to them than what is in style now. But some of the most notable characteristics of modern suburbs - winding cul-de-sacs, large lots and an absence of mixed-use - are definitely part of Don Mills. The fact that it is now considered close to downtown is merely perspective - compared to living in Richmond Hill, Don Mills is close. When Don Mills was built, Richmond Hill was not the commuter town that it is now. So things looked different.