Marco saying "By the time you come back the era may move a little more" is probably a red herring. It is meant to imply that BMPs won't make it to the battle.
But what will happen is that BM will now let the BMPs know about the port. So, at a crucial moment the BMPs will make into the battlefield and then Kaido will declare whatever the hell he wants to declare.
Since someone like Kaido is playing coy with this, you can bet that the declaration will be amusing if nothing else. This is just Oda's way of creating the tension around the declaration.