I don't know if I'm interpreting this the correct way but Oda is saying the lurking legend would be the greatest enemy for the Strawhats, has something to do with WB and is going to hinder to Strawhats in Onigashima. At first I thought Lurking legend and greatest enemy could be different characters but all these sentences seem to be linked with each other so it makes more sense that they are one and the same.
Kaido doesn't fit the lurking legend profile, nor did Oda not need to mention his name since everybody would have guessed that Kaido would be the greatest enemy in Onigashima? this is weird lol
Fujitora: "Does this mean The Rocks have returned?!"
Sakazuki: "Stop speaking as if they are a legend"
Now Sakazuki doesnt consider the Rocks a legend but in the world of OP they are a legend. And have always been in the shadows.
Sengoku: "The most fearsome crew the world has seen is about to take shape!!"
Aka the strongest crew, the bounties of who's captains surpasses even the Pirate King together. This would make them easily the biggest enemy for the strawhats. Makes sense, considering all the help the strawhats are getting.
"Related to Whitebeard" - Whitebeard was a member of the Rocks pirates
It's only weird if you were expecting it to be 1 actual person who was lurking in the world and then would come out of nowhere to be the greatest threat.
But it is the legend of the Rocks pirates themselves that still hasn't died yet, instead has appeared again with the alliance of BM and Kaido to form the most formidable pirate crew in One piece history.