If the Darkness is so all powerful and could have killed us, why wait and use the Vex as a distraction? The Darkness may be powerful, but tactics like this imply it isn't very smart. Personally, I think there is far more we don't yet know. The Darkness may be our greatest threat, but I also think we are the greatest threat to the Darkness. It's posturing because it doesn't want a direct conflict. It isn't sure it would win.