It is generally considered canon unless contradicted in-game, in which case the game takes precedence.
Lungrim was an infamous Zamorakian extremist and preacher of chaos. His teachings were so extreme as to be thrown out of by the more tolerant members of the Church of Zamorak, presumably during the time when both Saradominists and Zamorakians worked together before the burning of The Wizards' Tower.
He does not appear in game, and is only mentioned once in the Return to Canifis novel. The Charred Folk still follow his teachings, although they too were cast out from the church.