Actually, how on earth do they work the licence to use the Cycles code anyway? I'm pleased that it's possible but I've scratched my head over it a few times now. I'm just assuming that they link to the code internally somehow, but they still have to bundle it with Poser to do that, don't they?

That's the part that puzzles me, but whatever the method used, it would likely be true of EEVEE as well, to get a realtime viewport.