ATC primary radar can’t resolve objects that small, and the jetpack-person was unlikely to have the requisite transponder to make it visible to secondary.
I do actually own a lens that could do it. Extended to full 500mm focal length and mounted on a camera body, the whole package is about three feet (0.9m) long. That's a lot to wrestle in economy class, especially if you're not in the window seat! Given advance knowledge of the opportunity, with time enough to prepare, it wouldn't be too hard a shot to get, I think. But taken totally by surprise? Even if you spot me the camera being out and ready to shoot - which is no triviality given that this happened on final - and the jetpack guy being perfectly visible out the passenger windows, I'd still need a very large helping of good fortune, not to mention a good deal of tolerance on the part of whoever is in the window seat.