So I poked around at this and I think the issue is that the server wants a User-Agent header for some purpose. Most clients include one by default by App Inventor does not since we don't know what the final name of your app will be. For example:
After fixing that I did encounter a more serious bug that is causing an exception to be thrown when parsing the GeoJSON into features. I think it should be fixable as part of the next companion update.