Defeating another top tier is not a feat, we know that since the titles were introduced, Oda wants to insinuate for example that Whitebeard is capable of defeating any other TOP TIER in the world (with the exception of Kaidou),and for that he doesn't need to make Whitebeard face all the other TOP TIER in the world to prove he is above them and that he really can defeat them, just say that he is stronger for his title, just say that he is stronger through his title, the same with Kaidou, Akainu's case doesn't even compare with the titles and statements that Oda attribute to X and Y claiming to be the strongest in the world, because Akainu only defeated an admiral and that's it, what about the rest of the TOP TIER? They may well be superior to Akainu.
That's why statements that place Kaido and WB above all other TOP TIER is far, far superior to just beating a TOP TIER.
We know that from the beginning Kaido and Whitebeard can beat Aokiji and they don't even need to lift a finger to prove it, because Oda already does it automatically, while Akainu has to kill himself for 10 days against Aokiji to prove that in the end he is 0.5 cm above him.