Since React only runs on the editor and not the frontend of the site, does the correlation you made about the spike being from Gutenberg actually check out? I believe BuiltWith only uses data it sees and doesn't make assumptions about what exists behind a login. I could be wrong though...

I imagine the number of WordPress sites running React on the frontend to be extremely low.

The spike in React usage is more likely from the popularity of Next.js and similar frameworks.