I did a test and you are correct. It looks like it is deriving it from the hotspots on the HDRI. Interesting.
As to the other points, it is certainly true that the dome light is capable of doing a lot of cool stuff that goes beyond what the reflection/refraction slots in the env override does. No doubt about that. However in this case I am not trying to do that. I am simply wanting get environment reflections from an HDRI on materials. Not as a complete lighting solution, but rather as one piece of it.