User blog comment:B214/Question and Answers 6/@comment-190.164.227.239-20161201033222/@comment-9595671-20161206074514

@Crimson: If I may, I don't think Azazel said that as in he was the strongest at that point in time. I think he was saying that in the sense that he was sure Vali would become the strongest White Dragon to ever exist because of the potential within his Lucifer blood.

It's not that different from if someone said Issei after being revived by Ophis and Great Red would end up becoming the strongest Red Dragon to ever exist, and since Vali's LUcifer blood gave him his own Dragon DxD form, Azazel's guess was likely right.

So basically... you may have accidentally heard Azazel's words at present tense, but they were actually in future tense.