Perhaps they didn't know yet but were learning too much for the Speaker to risk it. You're the one who says Warlocks have the power to sense Darkness yet Taox is not the Darkness nor Hive. In fact Taox tried to eliminate the Hive before they even came into existence. Osiris gear very clearly tells us not to trust the Speaker. The Speaker grimoire card explains that they care much more about reviving the Traveler rather than saving humanity. Taox only cares about fleeing from the Hive and Darkness with the Traveler.