That seems surreal. Maybe by that point Stewart figured that anyone passed through to him had the technical chops, and the rest was about personality & culture fit?
This song in particular? Not much. The entire conversation, assuming it was more than 5 minutes of listening to the song? Perhaps whether or not you can get along with the person. I'm not saying that's what was going on: I think it's entirely possible that it was just some general weirdness. Or Stewart had already decided the person wasn't getting the job & wasn't going to waste time on tech talk when they could have a pleasant conversation about something else. I don't know, but I think there are other plausible explanations than the "weirdness" one. But massively wealthy tech founders are often an odd bunch, so "weirdness" certainly wouldn't surprise me.