Yes yes, the entire Echlisearchy is heretical in Bigs Es eyes, we know.
And wrong. Big E just told Russ to go watch Magnus, like he was under house arrest. Tzeentch garbled the message so Russ would burn the place to the ground.
Theres a few Chapters that do worship Big E as a god, but the vast majority just consider him the pinnacle of what it means to be human.