The source code for the post is available[1], as is the macro[2] which is loaded via XHR.
The short of it is that a load function is passed to `cljs/compile-str`, which provides the mapping. In this case, it doesn't bother to do any mapping because it knows that it is loading `bar.core`. However, `bar-url` could instead be in a map from the namespace to the URL. Surprisingly simple, actually!