It's not like Marco should have killed the admirals in the short clashes he got with them. He could block their attacks and send them flying without receiving any damage himself. Oden is dead while Marco is still kicking. Portrayal of first division commander definitely credits Marco more here
But it's not just Marco, it puts to perspective the strength of WB pirates as a whole, who indeed might have been the strongest yonko crew like i expected Oda to do lol