User blog comment:FairyRaven/Fairy Tail Chapter 356: Tartarus arc - Prologue, Review and Commentary/@comment-7505640-20131020033003/@comment-1359322-20131020040017

That's what Cobra's saying, they're all Demons not humans, even the leader is a Demon. Don't think from the words, think from the main context perspective. The main context here is that Tartaros consist of Demons from the Book of Zeref.