Earlier quoted context omitted.
The beauty of htmx is that it doesn't matter what you use on the backend. Literally anything that'll render HTML will suffice. Even static HTML files on a web server will do. No need for an application server at all if you're clever and your needs are simple.
What if I want the website to work even when JavaScript is not turned on? I'd need a backend server and some frontend logic handling that case.
You can make the left pane a list of links to dedicated pages, and then add an htmx attribute to indicate that when the user has JavaScript to instead fetch a html snippet with just full article and swap it into the right pane.
If you had a reasonably bounded set of articles you could statically generate all the /article/id.html and the /article/id/snippet.html files, though a backend probably makes sense.