If he's not a god, how did he rain a comet down on Mordheim? How are his warrior priests able to pray and have actual effects happen? You can't really say that's regular magic, as sigmarite prayers are totally untouched by Chaos AND the High Elves can't "cast" it.
Part of the point of Sigmar is he began as a regular human and became as a god through his own abilities. If he's just worshipped as a god, it doesn't make sense for prayers and blessings to have any effect, so there is something more than mortality about Sigmar.
Sigmar is awesome.
Although, Grimnir is pretty dang awesome too. He just decided that he was going to wander into the Chaos Wastes and kill as much as he could one day. And he did.
"Did you just deploy your guardsmen in ranks?
" - Quote from my last battle