They solve different problems.

Alpine is about defining in-browser behaviours in a nice, simple, declarative way.

HTMX is about defining data loading and brower-server interactions in a nice, simple, declarative way.

The author of HTMX have often suggested using HTMX and Alpine together for this very reason, they work very well together.

I think it's just an alternative https://www.libhunt.com/bigskysoftware/htmx

I use Unpoly (equivalent to HTMX for your question) + Alpine and they complement each other very well.

Unpoly handles everything that requires what in SPA world would be an "API call", the difference is that the response I get is just the HTML with the data already rendered and which is dynamically updated in the DOM. Also Modals and page transitions Turbolinks style are handled by Unpoly.

Alpine handles everything that doesn't require a server API call, such as form wizards steps, dropdowns, tables filtering, sidebars, buttons loading/disabled states, etc.

The end result, the application "feels" like Github.