I guarantee you guys that while Saint Shanks was parked off the coast of Wano, he sent one of his Fishman crew members down to the bottom of the Ocean to make a rubbing of the Road Poneglyph.
There's really no other reason Saiyuki would mention the fact that Kaido found that Poneglyph due to the fact that Jack is a Fishman. Plus Saint Shanks declaration that he's going after the One Piece kind of solidifies it.