User blog comment:IamJakuhoRaikoben/Chapter 433 & 434: Ikusatsunagi & Demolition Fist, Review./@comment-24546844-20150511182459/@comment-1776824-20150511222255

Having your soul stolen does not equal defeated in terms of power. Acnologia taking their souls simply translates to they did not know that he could do that, and it left them all power-drained. Igneel nearly defeated Acnologia with nowhere near his full strength. If Igneel had been full strength, he could've defeated Acnologia, and this was justified with the soul-taking explanation.