Oda isn't the god of foreshadowing but of literary sharpshooting, that is to say, he loves to leave potential targets throughout his manga that he may develop or not, so when he actually does create a plotline out of one it seems like he shot a Chekhov's gun he had prepared in advance and hit the target. This also helps him to keep his manga alive by feeding the fans with tons of irrelevant elements that feed all kind of theories, such as Trébol's shackles that pointed to him being a slave when Doflamingo lived in Mary Geoise but that ended up meaning nothing.
Kaido's title shouldn't be taken as a literal statement but as a way to stress his inhuman nature. Oda has never meant him to be the strongest individual but a thing that is closer to Big Mom than to Roger and Whitebeard in their prime.
Kind of following the previous statement, people take titles in One Piece as if this was a static videogame instead of a "living world" and Oda may easily see many characters who use a sword as stronger than Mihawk no matter his title because they're ultimately irrelevant to Zoro's journey, never met Dracule, came into action recently, and so on. Just like Whitebeard kept the title of World's Strongest Man in current time only to assist Marineford's narrative of a new gate being opened, the legends stepping aside and Whitebeard himself admitting he can't be the strongest forever.
Oda isn't a fighting guy and deep down he's disappointed at his own fans as they spend more time discussing power levels following absurd premises such as "mountain level attack" instead of researching all the references in his work: Hercule Poirot, Sancho Panza, Howard Pyle's pirate, Yatsukamizu and so on.
Foxy's arc is a good way to establish the themes of the upcoming arcs, as he represents a relatively harmless enemy that breaks the crew in a joking way, only for the Straw Hats to face an unstoppable force that actually threatens their lives (Aokiji) and lose crewmates in a serious manner (Usopp and Robin).