He was never Swiss. That's an anachronism. The Principality of Neuchatel, which during his lifetime was ruled by the Prussian. It didn't become part of Switzerland until the century after he lived. How could he be Swiss?
He is identified as Swiss in everything I have ever read on the topic. The Encyclopedia Britainica labels him as a "Swiss Jurist."
Perhaps he regarded himself as Swiss, perhaps not. Perhaps Prussia paid little attention to Neuchatel, and it was effectively Swiss.
I very greatly doubt the Prussian King would have tolerated his ideas about overthrowing Kings, because those Prussians tended not to have a very good sense of humor about challenges to their authority. Maybe Neuchatel was autonomous at that time.
So you got me. He might not technically be Swiss, but he is identified as having a Swiss Character, and he is constantly identified as Swiss.