It would also explain how they could reconcile the differences between the PPC floating point length and the x87 code on Xbox One/X/S for specific titles.

They aren't so much pure emulation as they are binary conversions with intermediate interpreter for API calls (GPU/APU/IO etc). Combine this with splitting the 6 threads across 6 cores instead of the original 3 cores and this is how they could get reasonable performance out of the Jaguar chip.

There would be a lot more to it than that but that is just my basic theory. Would also explain the slow roll out.

I think this is the same thing they would have done getting the original Xbox onto the 360. But just a theory.

All of the original publishing contracts allowed Microsoft to sell Xbox/Xbox 360 games on those systems specifically, so Microsoft had to seek permission from each and every relevant third party publisher before making each title backwards compatible.

From what I understand original Xbox compatibility on the 360 was similar, with 462/996 games ultimately being compatible "officially" (https://en.wikipedia.org/wiki/List_of_Xbox_games_compatible_...). But with a jailbroken system compatibility is actually a fair bit better (https://consolemods.org/wiki/Xbox_360:Original_Xbox_Games_Co...).

Afaik there is no similar jailbreak for Xbox one/series, but if anyone ever does manage to get their internal emulator running with arbitrary titles I wouldn't be surprised if a lot more games work than their official numbers suggest.