Again, It's Blizzard. Never ever rule anything out. To the point of him not being present for legion they could simply say he's been reforming on Argus, since it clearly takes time to do so. (See Mannoroth dying 30 years in Draenors past and not being revived until we get there).
They could also say that he and KJ are super special and give no shits about where you kill them because Sargeras powers.
It's stupid, but so is the idea of fighting a dude who can cut planets in half. At least dramatically having KJ / Archimonde / Velen together in one encounter would be interesting.