But you're still doing the work. With React, you render in the client. With HTMX, you split rendering between client and server.
On the other side of the scale, React has overheads like a virtual dom.
It is a complicated question as to what is most performant, who bares that cost, and so on.
The issue is overhead and complexity for the developer, and maybe, server costs.
React comes out ahead in server costs, since doing work on the client is very cheap (provided there isn't a performance issue that turns users off, which there isn't).
And in terms of dev time, it depends, but you can't say "look, with HTMX you can avoid code" because you still do need to write code, but now it's in the server.
All HTMX is doing is fetching HTML snippets from the backend and patching them onto the DOM.
That's how it should work, although you can get easily get around that and tack on client side rendering.