Is it the fact that he's a fraternity member or that he used the frat shorthand that bothers you?

I would say, like basically all groups of people, fraternities contain both good and bad people. They are also organizations that vary wildly based on the school they are located at. Without a dozen more data points about this person, I doubt it's safe to come to any conclusions about them.

He mentions that he used to work in investment banking. I'd say that makes it no surprise he's in a fraternity. Besides, investment banker throws a bigger warning my way than fraternity. But I'm sure there are some really nice bankers out there who would prove me quite wrong.

Yes, it's the shorthand. By every account I've heard, it's always, "fraternity brother." The standard rebuke is, "do you call your country a...?"

And I have to say, the red flag was whether this was even real, or how much. Investment banking doesn't touch that for me.