You are comparing apple to oranges :)
alpine and stimulus are frameworks (doing all sorts of stuff: css animation, data <-> objects, etc) while htmx is a library focusing solely on ajax.
From your list, the only similar project to htmx is intercooler-js, but... htmx is actually the new version of intercooler-js (both projects by the same author).
htmx supports animations too, and it has extensions to add further functionality. Of course those projects differ in their functional range, but they share the way they work; by adding attributes to HTML.
Yes, you are right. My response was based on the presentation from the first page: "htmx allows you to access AJAX, WebSockets and Server Sent Events directly in HTML, using attributes". In the meantime I read the documentation and, as you just said, it supports animations and much more.
https://github.com/stimulusjs/stimulus
https://github.com/alpinejs/alpine
https://github.com/intercoolerjs/intercooler-js