I really don't think this island looming over the capital scenario will be the ticking time bomb of this arc because of this-
What does he intend to do then, ask the citizens to evacuate and just dump it there? I think this should be resolved by 998/999 at best.
Before Kaido and BM are inevitably interrupted by Kid and Luffy, they could talk about their time at Rocks or give us some insight about Xebec.