Related questions:
If a PC builds a stronghold on behalf of his hench, does the PC or the hench get the Construction XP?
If all Henches get half XP for magical research, and everyone in the kingdom is part of a web of vassaldom, does that mean that only an emperor-archmage is capable of getting full XP for research?
Not an Autarch, obviously, but…
Vassaldom is not necessarily henching. You can have non-hench vassals. But hench vassals are usually the most loyal.
So, if your empire is made up ONLY of non-PC henchmen vassals, then yes. But if any of the vassals are PCs or NPCs that are not your henchmen, then they would get full XP.
My two cents, anyway.