User blog comment:FairyRaven/Fairy Tail Chapter 407: All for My Demise, Review and Commentary/@comment-24490201-20141101160852/@comment-7418318-20141102010323

If Igneel actually fought END, it's obvious that at that moment END wasn't sealed yet. If Igneel was forced to to retreat and someone/something else sealed the now weakened END, Igneel might never have learned of it.

But you brought me to another idea. Maybe it was Igneel who placed the seal (maybe with the help of someone else like Grandeeney) and because Natsu is his son, he could undo the seal.