"Never before have I heard that Southeast Michigan and The D mean the same thing."

A bit over the top don't you think? His perspective is of the person living outside of Detroit and how the city is promoted outside of SE Michigan. What we see or hear inside the city is irrelevant to his point. It's the image that the image makers are trying to project to the outside world. From what he's seen and heard and what you see here:

http://www.visitdetroit.com/index.php/summer

or here:

http://www.crainsdetroit.com/section/lid

"The D" is thrown around as an alternative to "Detroit". That's his perspective. Unless you spend the majority of your time outside of the city and area, I'm not sure how you think you can speak better about an outsider's perspective than they can.