I doubt it: he explicitly denied it in a blog post, and it's not clear how good he is at programming and system design (for all that he's obviously smart). More speculatively, I kind of doubt his personality as shown by his writing is that compatible with maintaining Nakamoto's personality as shown by theirs.

Also, your prior shouldn't be too high: although Szabo is well-known from his writings now, they didn't come out of nowhere. He previously worked for Agorics, Inc., which was founded to develop these ideas: https://e-drexler.com/d/09/00/AgoricsPapers/agoricpapers.htm... which I think of as more fundamental than Szabo's smart-contracts paper. (Admittedly I never finished slogging through that one.) If you think of smart contracts as emerging from nowhere then it probably seems more likely for Nick Szabo to be the lone genius behind it all.