As one of the other replies mentioned, the fetch will throw an event that you listen for. The server is the response formatter. The piece of the puzzle that you're missing is that the intent is that this library is used with the author's other library, htmx[0]. With htmx, the expectation is that you're always returning HTML as the response (though there are extensions available so you can return JSON that could be processed against a client-side handlebars template).
[0] https://htmx.org