Fuji explained it already:
I'm glad to have you aboard Baby.
Answer my question. What's holding Zoro back from being a low top tier?
In all fairness, this isn't a straight 1 vs 5. The Supernova are exhausted and pretty injured:
- They described facing Kaido and Mama as going through "Hell".
- Luffy said he's been to Hell and back a few times already.
- This chapter is named "Hell".
- Luffy, Killer, Zoro and maybe Kidd were all shown heavily panting last chapter.
- This indicates their exhaustion.
- Their heaviest hitters took major damage from the Yonkou:
- Luffy took Kaido's Three-world Ragnarok head on.
- Zoro withstood Hakai for a bit before Law saved him.
- He was stated to be heavily injured afterwards.
- Later on when he cuts Prometheus, he's coughing up blood.
- Kidd took a beating from Mama
- Redon outright said that the Supernova were so damaged he didn't think they had the ability to beat Kaido in ten chapters let alone one.
The Supernova are far from their best, while Kaido's Zoan recovery keeps him relatively fresh. Besides, Mama may yet return.