You are comparing apples to oranges. The author could probably couple their library with https://vite-plugin-ssr.com/ and have something comparable within a day.
The reality is most running React apps today are still traditional SPAs, and chances are likely the same apps can be rewritten with VanJS (or any other SPA framework) and the users would not notice a difference.
They would notice because vanjs trashes the entire DOM at the root of every state update so users will lose focus, animations will interrupt, and big perf hit layout reflows will occur.
The reality is most running React apps today are still traditional SPAs, and chances are likely the same apps can be rewritten with VanJS (or any other SPA framework) and the users would not notice a difference.