Originally Posted by dabac

OK, fine. A part of the hub that isn't supposed to come off has come off - in a novel and unexpected way. No questions there.

And if it does say Shimano on the item then I'm not going to argue with that.

But the protrusion you've circled is way too short to get a solid engagement, if that's all the protrusion you can get from the body fixing bolt. That can't be what was meant to hold that thing together.
Although the bicycle industry does come up with some rather sloppy engineering from time to time, something like two turns steel into aluminium would be rich even for them.

I'm still thinking that the important threads are on the inside of the splined bit, which was meant to be a press fit into the central body of the hub. The threads you're seeing in the hub are probably just runout from running the tap past the maximum insertion depth. My hubs have plenty of that.
If splined nub is a good and tight fit into splined recess, I'd still be ready to try a repair - to save the hassle of a wheel rebuild. A good clean, a good metal-on-metal glue and you're all set.