It contradicts nothing whatsoever. We were never, ever told that we'd fight Sargeras, that was an assumption. We were never ever told, that Argus did not have a world soul, we were told that Draenor DID NOT have a world soul, and told, that worlds without souls are rare, so rare that Aggramar himself went to explore it and by that logic, Argus should have a world soul. Hell, Kil'jaeden's words in the 7.2 trailer makes much more sense now that we, the players, know that Argus has a soul too.