Why is "dynamic web app without page reloads" not the same domain?

That's certainly part of the domain of react applications. But there are a lot of react applications that aren't (really) in the domain of HTMX, like "local first" applications or other applications that rely heavily on browser js APIs.

Absolutely, but there's a huge overlap in terms of what react and htmx can serve. Most websites that use react, can just as easily use htmx.

It's a little like comparing HTML and JavaScript, isn't it? React is a JavaScript library (in the sense that you write your own code that calls React functions). Whereas HTMX doesn't integrate with your application logic at all. It provides a mechanism for placing your application components on the screen.