U of M still has a Dearborn campus that's closer to Detroit, doesn't it? Looking at Maps, it's only a few miles from downtown Detroit. I grew up in Flint and the U of M campus there was a big part of the city.
UM Dearborn is largely a separate entity. They're related but when people talk about UM it's not what they're referring to. It has its own much smaller student body and faculty, etc.