User blog comment:B214/Chapter 499: Gray & Juvia/@comment-5303512-20160829121430/@comment-1359322-20160829134248

You miss the point magnificently. You focus on the point that it was before Keith was revealed as a Necromancer. Then let me ask you this, why did Silver say that disgusting name when Natsu said Gray? There's no reason for Silver to hide anything at all since Mard is also well aware that Silver is a corpse.

My point is that Mashima can just change things as he continues writing or in this case, there's nothing that guarantees what Zeref told Natsu in the Tartaros arc will remain the same as he continues writing. He can change it anytime he wants.