Semantics over the name aside, you end up returning small chunks of HTML from newly defined routes that are only useful for your HTMX frontend, hence my usage of API as the interface really only makes sense to the machine and not the user without the rest of the page content.

Yes you could inspect the headers and such to determine what type to return, but the core problem remains, that the HTML in those is not reusable whereas as JSON would be.

How much overhead this is for you depends on your design of course, but it does mean some work might potentially be wasted if you go down the full SPA application later and even if not changing layouts and such might be a pain.