Is it really that hard to credit Graham McNeill with some intellect?
He wouldn't have made a mistake like that, considering he's one of the people who established the Horus Heresy timeline way before the novel series was ever a thing.
The whole point of the Outcast Dead / Wolf Hunt timeline is to drive a point across.
Magnus wasn't punished for actually playing around with minor spells, whatever bans Nikaea stipulated. He was punished for practicing sorcery on such a scale that it not only lasted two years, managing to overcome the most formidable array of psychic defenses in the Imperium (defenses which, I would assume were being constantly bolstered throughout those two years), but also caused considerable morale damage among the population, when it eventually broke out (IN the middle of a Galaxy-spanning civil war).
I couldn't agree more.
"We've been looking for the enemy for some time now. We've finally found him. We're surrounded. That simplifies things."