Suburbs aren’t necessarily low density, though they usually are. In places like New York, many “suburbs” are just small satellite cities well connected to the downtown core with rail.
Plano is a city of 71 sq miles and ~3800 people per square mile. Westchester is a _county_ of 500 square miles and 2300 people per square mile. In many regards, Westchester may be denser. It also looks like they haven't chopped down every last tree and laid waste to the land with asphalt.
Never been to Westchester, except on a train passing through, but I am intimately familiar with Plano. It's dense, but most of it is a poster child for shitty suburbs: if you don't have a car, you're fucked. (Take a bus, you say? What bus? When I was living around there, most of Plano wasn't served by buses.)
Those are not the example of suburb that people are referring to when they discuss the sustainability issues.
Usually they are referring to the much sparser developments of often large houses, built between the 1960s up to the early naughts.
What you are describing are more like towns, which usually have more sustainable features (smaller houses and lots, ergo less pipes in the ground, a rail corridor connecting them to a major city) because they were built before the car era.
