That’s Argus, not Outland. We “killed” Kazzak in Outland, yet he returned. So either Outland isn’t completely seeped on Fel, not fully immersed on the Twisting Nether, or most likely for you to belive, another inconsistency.
With so many inconsistencies of Outland not being seeped in fel to the point every race should get corrupted, maybe it’s just not as seeped in fel as you think.