I've seen this misconception a lot, but HTMX and Alpine do two different things.

Alpine enables client-side reactivity and does not require a round-trip to the server. Say, clicking a button and incrementing a counter on the page.

HTMX enables something more akin to server-side reactivity. You could use it to modify, say, a single element in a list of elements. The crucial difference is that HTMX makes a call to a server. This means you would use it when you need to read/write data to/from the server.

Alpine provides client-side reactivity. HTMX provides server-side reactivity.