Tbh considering how they still haven't been able to decipher the logia,it doesn't make sense. Not too many chapters have passed since Vegapunk told us that.
Unless somehow in the series timeline in the past they thought Mythical zoans are the strongest because they are the rarest. It doesn't make sense how he is flip flopping.
Did Oda decide to change his tune because he is going to introduce more and more enemies with Mythicals? It certainly seem that way .
He opened the can of worms with the mythical of Luffy,so he thought he will have more freedom in the powers of Mythical Zoans .
In comparison there aren't a lot of powers to explore in Logia.