Unhappy Anchovy is a liar. He talked about the Emperor not wanting to kill all aliens and left out the part in Horus Rising that shows Abaddon talking to Horus about the Emperor's "suffer not the alien" policy when Horus talked to the Interex.
Magnus sent an avatar in Battle of the Fang, as he is too powerful to exist fully in the materium. So why do people bring up him in the Fang as though he was at his full power (all Daemons are stronger inside the warp and have to use avatars outside of it)?