I'm a random dude on the Internet, but my partner completed her PhD at MIT. While there I knew and knew of a few PhD grads who worked at MIT in some non-tenure-track role (postdoc, staff researcher, etc). Typically for a couple years and then they get a better-paying or more permanent job. But several remained "affiliated" in some way. They kept their MIT website/email, some in academia continued to collaborate to some extent. Things like that. But AFAIK they weren't getting a paycheck from MIT. And it's somewhere between neat and genuinely professionally valuable to be affiliated w/ a prestigious university, so I don't blame them for claiming affiliation. My best guess is he's "affiliated" in a similar way.