I think he’s referring to its use as a rhetorical question, often used as an expression of mild surprise. I’ve always connected it with Jewish American patterns of speech but I’m probably way off base.
That’s it exactly: mild surprise. Mild surprise expressed somewhat humorously: “So, you’ve heard that Maria ran a marathon on Sunday, in pretty good time, too! And she’s such a heavy smoker! Who knew?” I also have a feeling that it is Jewish American in origin (there’s the hint of a Woody Allen-ish shrug about the phrase to my ear) and was actually wondering whether anyone else could corroborate that. Interesting that you would have the same intuition, aldiboronti.