In One Piece a lot of a characters standing is based on reputation, not just power and WB undoubtedly had the highest rep out of the Yonko
And instant hype in manga shouldnt be taken absolutely literally, every villain ever in manga gets an instant giant dose of hype because weekly manga just doesnt have time to set it up any other way, even if Kaido is the strongest man alive (which is likely) it doesnt mean he is the top combatant in OP, I think in his case strength is meant literal physical power ... there are things more broken than pure strength, many devil fruits are broken, haki may be broken, Kaido is the big thing now, even if the arc is about BM, Kaido seems to overarch the last few arcs, but after him we will get a fresh batch of uber hype about whoever comes next, be it the WG, the Navy, BB ...
What BM meant about WB in the last chapter was most likely based on reputation, WB and Roger had their emtire age to attain god tier pirate reputation, but we have a new generation now and there is no reason to think that the top dogs of today couldnt rival them, and imo Oda doesnt do "strongest character" because thats horrible and boring, just look at Madara, it will also be the same with Luffy, he wont just be the strongest character, BB will rival him, probably someone from the navy will rival him too, maybe a few others will too ... Law? Maybe Sabo? All i know that for OP to be a good story and stay interesting we need a tier of top ppl and not one definite strongest character