It may have been something as little as Sexton calling him and saying “Hey, Saban is retiring, will be announced soon. You’re on Alabama’s short list. Will move very quickly, be ready on a moment’s notice and start having conversations now with your family to confirm your interest. The terms will be probably be somewhere around XYZ, talk soon.”
Who knows exactly when this may have happened, but there’s no way it didn’t happen sometime before the title game. Maybe even as early as November or December.