User blog comment:Crimson and Darkness/Shin DxD 3 discussion/@comment-82.160.23.173-20191230163157/@comment-24595619-20191230230509

Hope this helps, this is what Ichiei Ishibumi said in the afterword of Vol 13:

In volume 12, I wrote that "He has the power that is equal to the whole Yondai Maou together", but I received several questions from readers who read this and asked me "Is that the comparison when you include Sirzechs in his true form?". The true Sirzechs and the serious Ajuka (who haven't appeared in the story) isn't included in the comparison. If Śakra is stronger than the Yondai-Maou when you include Sirzechs' true form, then the power influenza will be go off.