You can just respond back anything and it'll work. You can just respond a flat non-hypermedia JSON and have some JavaScript listen to the event and process it with whatever semantics you want (although it'd be weird for you to do that, but I just tried it and yeah it works).

I still don't understand what they mean when they say "HTMX is HATEOAS": it's not parsing the response to decide things based on relations, it's not following any links, you can still have out-of-band info if you want...it's just substituting a string given by the server on the DOM.

A JSON API can be as hypermedia as HTML (I remember HAL-FORMS and Collection+JSON). A web app that consumes such an API is doing "HATEOAS" even if it's using React.

I really like the HTMX project (that and Unpoly are a godsend) but sometimes it sounds more grandiose than it is: just the regular web doing what it always did, except you program the interactive fetching/swapping of pages/fragments in HTML instead of JS, i.e. AJAX with HTML attributes.