Statements are all we have for top tiers though lol. You can just discard any statement made in any manga by using the "it's just the opinion of X character", it's a terrible argument. It's portrayal being given to us by the author. Now when we try to argue certain characters over other, then yes we need feats but statements are more than enough to prove they're atleast in the same ballpark.
Gaban will defo not be stronger than Rayleigh, but this statement proves they're in the same ballpark and it's the author's way of telling us how strong they are, it's called portrayal.
So if Gaban wants to prove himself over certain characters then yes he needs feats. But this statement is more than enough to prove he's atleast on the same level as Rayleigh