5th Generation Slayers are a completely separate generation.
To those who are unable to grasp the concept still:
1. A "Generation" of Dragon Slayer refers to the way one obtains the ability to properly use Dragon Slayer Magic. First Generation obtain their powers from learning (or being enchanted by) a Dragon, Second Generation obtain powers from a Lacrima, Third Generation are incompletely taught by Dragons and require Lacrima to achieve their proper output, Fourth Generation are "Dragon Slaying Robots" (so-to-speak), and Fifth Generation are those who obtain their powers by eating Dragons in the first place.
A First, (possibly) Second and Third Generation Dragon Slayer can enhance their powers by eating Dragons, in the same vein Acnologia was assumed to do. However, that does not make them a Fifth Generation by default, as they possessed existing Dragon Slaying abilities.