Two things:
1) The yonko commanders were severely injuried by zoro and sanji. King in particular got his wing chopped off. In no way, shape or form are they close to 100% health. Just look at luffy and zoro, they were the winners and they literally just woke up. Now imagine taking even more damages than they had.
2) Greenbull's entrance is similar to Magellan's entrance. Sure Magellan neg diff the entire BB crew and BB himself, but that is mostly due to his hax DF and how they were not aware of it. No one in their right mind would think that BB + his crew < Magellan. This is very similar to Greenbull's situation. We all know that admirals all have hax DF.