User blog comment:IamJakuhoRaikoben/Chapter 470: The Hybrid Theory, Review./@comment-27674224-20160126181825/@comment-7418318-20160130093804

How does it not confirm it?

From Serena we pretty clearly see that Acnologia includes 2nd gen DS in his count and he said that after Serena's (supposed) death there are only seven left.

Unless Cobra was killed off-screen or ripped out his Lacrima (both are astronomically unlikely) we know who those seven are, so there's no room of Invel to also be a Dragon Slayer.

Or do you truly believe that Acnologia was able to smell out 8 Dragon Slayers over two continents and miss one who's just as close to him as Serena was?