Why not? AFAIK it will load http://localhost/, is this the exception?

Yes, localhost IS the exception. There is a lot of code that treats "localhost" specially. IIRC Chrome will let you do things on localhost:XXXX that are normally only possible for sites over HTTPS (Think geolocating and some other privacy-related things).

iOS does load it, for example http://ai/ opens just fine in iOS

This only opens if http://ai./ has been loaded before. Otherwise you get ‘host not found’.

I...what?

That's really impressive, that between the choices of "yes it works" and "no it doesn't work" they managed to make a third options that's far worse.