I'd say alpine is more front end oriented and better compared with vue.js or maybe react. htmx is for driving server interactions via dom events, within the hypermedia network model.
The htmx and alpine pair very well together, we have a lot of folks on the htmx discord who are passionate about the htmx/alpine/tailwinds, so you can do all your work in HTML (locality of behavior[1]).
The htmx sibling project, hyperscript (https://hyperscript.org) would be a more direct comparison with alpine, but it isn't reactive at all, so it would be better thought of as a jQuery replacement for light front end scripting.