I don't know what you mean by an API standard, but yes, it is technically a JavaScript library. But that's only an implementation detail and the spirit of htmx is that you write very little JavaScript. Javascript is simply used to extend the HTML standard to support the full concept of hypermedia for interactive applications. An htmx-driven application embraces hypertext as the engine of application state, rather than the common thick client SPAs hitting data APIs. In such a model, clients are truly thin clients and very little logic of their own. Instead, view logic is driven by the server. It has been around for quite a long time and is very mature.
It's fundamentally different than most JavaScript libraries out there, which are focused on thick clients by and large.
I was replying to someone talking about GraphQL and Ruby on rails, not the OP of this post.