N'Zoth is the only Old God left. That was Hearthstone blog entry, not written in-universe. Yep, canonical lore book doesn't count because you say so. You realize in-game materials are also contradictory? But sure, WoG, multiple separate sources... clearly it's all just fanon.
EDIT: Also, Il'gynoth is part of the Legion? Must be why it calls out to N'Zoth when it dies...