> This looks like a script element being created and appended to the document.head: https://github.com/jquery/jquery/blob/master/src/ajax/script.js#L34-L48 which would be invoked as a result of this call site: https://github.com/jquery/jquery/blob/master/src/manipulation.js#L335-L345
> Is that incorrect?

Yes, it is incorrect. Transport selection is a bit more involved than a simple one-on-one correspondance with a dataType. You missed the condition here: https://github.com/jquery/jquery/blob/master/src/ajax/script.js#L30

If it's not a crossDomain request, then ajax will continue searching for a suitable transport, until it reaches the one in `xhr.js`. Effectively enabling synchronous same-domain requests. Executing scripts synchronously is mandatory for `domManip`. It is, and always has been, broken for scripts of another domain but that's beside the point.