As expected, there's been some pretty vehement denial of any Deceiver/Cegorach link. That's fair enough, as the two are clearly not the same.
Still, you have to consider why the two would be linked. Firstly, let us remember that we know the Necron War almost entirely from myth. Did Cegorach really trick the Outsider into consuming other C'tan?
Probably not. The Eldar race was in its infant stages, and it's highly unlikely that they would have affected the Warp to that degree. The calmer nature of the Warp at that time, in essence making it more 'suggestible', might have made it easier for Eldar deities to emerge, but it still remains very improbable that Eldar gods would have arisen, and arisen strong enough to fight the C'tan and win.
The Eldar of the Necron War period had very little technology, let us remember. As Inquisitor Horst explained - 'They fought with swords, spears and their own twisted version of faith.' Mind you, Horst was retelling the Eldar myth, which is itself unreliable. (And says some very questionable things in the same passage - notably the creation of Wraithguard, which rely upon non-existent spirit stone technology, and Necron War-era Necrons being more powerful than modern ones. It's because the myth is apocryphal, I suppose.) I guess it's possible that the Old Ones would have armed their cannon fodder with weapons capable of harming the Necrons, but that doesn't fit with their ethos.
In any case, the Eldar of the Necron War period were restricted to a single planet, and did not exist in anywhere near the necessary numbers to create Warp deities on the scale of Cegorach, Khaine, or Asuryan. As Eldar religion evolved, they likely assumed their gods to be eternal (a fair enough belief - I daresay most followers of Chaos believe the same), and retroactively added them to the myths. With word of mouth, things change and details are lost - and there was a very long time for the details of the Necron War to be forgotten. Especially after the Enslaver Plague, it's very unlikely that the Eldar would have any real records of that time period. Perhaps much of their knowledge came from the few records the Old Ones left behind, discovered long after the Necrons were in stasis and the Old Ones extinct?
Okay, so, three of the Chaos gods were born of humanity, which only inhabited a single planet at the time, but we can assume that events there were somewhat different. There was a whole galaxy of pain, rage, despair, and ambition around to fuel the birth of the Chaos gods at the time - Terra merely provided the spark. The Chaos gods are quite broad deities, and draw power from all creatures who feel certain emotions. The Eldar gods were a racial pantheon, specifically attuned to the thought patterns of the Eldar race. Without sufficient numbers of Eldar, they could not have been born.
Anyway, based on all this, I think it is highly unlikely that Cegorach actually did trick the Outsider into going mad. Cegorach just didn't exist, or at least not in sufficient power to affect the Outsider like that. The Deceiver was wholly responsible for the C'tan-eating-C'tan incidents, which drove the Outsider mad. The Eldar later ascribed this to Cegorach mythologically, regardless of the real truth of the matter.