Its just a theory that's been around for awhile now. It would explain why Acnologia is so strong.
Yes , it is good as a theory......but major flaw is ACNO has never absorbed souls.The reason behind the strength of ACNO is his over-use of DS & its side effect......
Besides, Zirconis himself stated all the dragons in that graveyard were killed by Acnologia.
Otherwise, he would have told Natsu and the others about the soul riping technique.
What we saw was a soul that hadn't passed on...sort of like a ghost with regrets.
Zirconis soul was only powerful enough to stay there for a few moments due to WENDY's powers.....Similar was with ATLAS FLAME.......Moreover ZIRCONIS soul was only lingering there about to pass this world......Yes , what he told NATSU & others was correct.....ACNOLOGIA killed them through DS....He did not have enough time to specify the method......Even still it is not specified except the fact that he ripped their souls using DS.....No absorption of souls.
Also, we still don't know much of Larcade's magic....only that he can rip out the souls of humans.
So why not dragon souls as well.
Yes , it is true.We do not know much about LARCADE's magic.....But there is a similarity between ACNOLOGIA's DS & LARCADE's magic.....Both involves soul ripping....One is DARK magic while other is LIGHT magic........I think this is what ZEREF meant by "counter to ACNOLOGIA".
During the fight against him , LARCADE can help ZEREF by preventing ACNOLOGIA from ripping the soul with his magic...or specifically the magic.
Yes , Dragon Souls are good theory....But I find this major flaw of souls absorption...I can be wrong..

As for the source of ACNOLOGIA's strength , as
@kira says , I agree he has killed a lot of dragons overusing DS as he had no antidotes like current DS.......and bathing in the blood of too many dragons thus killing them.......