If you render everything with JavaScript then HTMX is not really going to be useful at all. It's useful for developing apps that simply don't need the sort of complex client-side interaction that those frameworks enable. The article points this out- if you want to write a SPA in React, HTMX is not going to help at all. But if you want a mostly-server-rendered page with some AJAXy interactivity, HTMX is much much easier than trying to shoehorn in some client-side rendering just for the AJAXy bits.

The main thing is is that there is still a substantial demand for tools that let you mostly avoid writing lots of JavaScript, whether on the server or the client.