User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-125.212.122.245-20150518133001/@comment-9595671-20150518143333

And it seems he met Igneel later, its heavily implied that Igneel didn't fail to destroy END because he simply didn't kill him. He probably realized something, plus Grandeeney was about to tell him something related to Zeref, we never learned what it was though.