Adopting something like HTMX is primarily a UX choice, and the language you use to serve it is almost irrelevant to UX. JavaScript is a nice backend language. It’s fast, has a massive ecosystem, and many people know it well.
With HTMX, you get these benefits regardless of what language you use on the server. These benefits could be reasons the parent commenter chose HTMX, and JavaScript is a perfectly good language to serve HTMX applications for the reasons I mentioned.
Next.js and TanStack Start don't send any JavaScript to the client if you decide to server-render everything. The upside is that you can opt-in into client-side components, and have all the interactivity benefits of React. I understand using HTMX if you are building a backend in Go/Python/..., but if you are already using Node, I see no reason not to use TanStack Start instead.
AFAIK, this isn’t true. And you still get more JS per interactive element than with HTMX.