Because passport control is not an indicator of whether a flight is domestic or not. France has many DOM/TOM overseas that involve passport control, but you are still in France whether you are on Réunion or Martinique- even if you earn Long Haul credit. But it's technically a domestic flight from a pure technical standpoint.

Taiwan, HKG, and Macau are part of China, and therefore count as domestic flights. But these entities retain their separate systems (or in the case of Taiwan region, its own central gouvernement) and thus immigration policies are still owned by these separate entities despite them falling under the same country.

I received 2XP flying between PEK and TPE on CI, which is in line with what the Flying Blue rules dictate.