But the fact that it is a world's “top class” battle is why this is so dangerous.
You must understand... that this order is given to prevent what might happen.
This could relate to Kaido's comment on Luffy's smile. Maybe Joy Boy will awaken once Luffy finally defeats Kaido, or he will be close to it and that's what the elders want to avoid? As if that's when he reaches the max of his smile...
This line is also interesting because Marco already mentioned that Onigashima wasn't named like that before, so maybe he knows more about the island; either this or it's just empty hype from Oda since it reminds of other statements about the strength of the Beast Pirates:
Marco: "Don't be distracted yet.
We are on the most dangerous island in the world!!"