User blog comment:FairyRaven/Fairy Tail vs. Alvarez Empire! Place your bets!/@comment-112.211.107.172-20150916005228/@comment-7418318-20150916111335

Ivan could just be the indirect source of the knowledge. We know that Obra (assuming that's the name of the Demon, rather than the doll it was inside of) is one of Zeref's creatures. It's quite possible that it might have stolen the knowledge from Ivan and passed it on without Ivan ever knowing about it.

Or Ivan might also have traded that knowledge with Zeref or some representative of Alvarez for certain favours... whatever those might be or have been.

It could even be that Obra only served Ivan for a time as compensation for the information that Ivan provided.