In my head, a static page is something that's served verbatim, without some backend generating it or inserting things into a template.
Wikipedia seems to agree with that view: https://en.wikipedia.org/wiki/Static_web_page
CSS and JavaScript and images are also static, it doesn't have to be just HTML
If you have a perl or bash or c backend generating html in response to http requests it's not static
The combination of all of these direct a browser in how to render a page.
Traditionally, a static site or static page is one whose data can be delivered to the client directly as stored, with no server-side alterations or generation.
This is/was a meaningful distinction because a server that can stream stored data is fundamentally much simpler than one that executes programs. Such a server can run in different contexts, be optimized in different ways, and satisfies constraints that allow further optimizations downstream.
Otherwise, all of the React server side rendering is static HTML.
The issue is whether they are being generated dynamically by a server. This project isn't. You run a build-tool once and then can post the .html page anywhere. It's static.
If you pre-compile the HTML using something like Jekyll, so that the webserver is just serving HTML files without any dynamic/on-the-fly processing at request time, then it's considered static.
React on the client side: static, as you still got the same files served, how they dynamically alter the page runtime doesn't count.
'Static' means it's only made of static files that can be served by just a vanilla Web server (i.e. just by serving static files).