Zoro has been nerfed, in the sense that he's ignored offensive opportunities to focus on protecting Luffy:
- He only launched Hiryuu Kaen as a counter to Kaido's Boro Breath.
- He only launched Kokujo Ou Tatsumaki to rescue Luffy after Kaido had chomped on him.
- He ignored the challenge Kaido directly posed at him with Tatsumaki: Gale of Destruction.
Kaido challenged Zoro directly with Tatsumaki: Gale of Destruction. Zoro could have tried to counter that attack. Instead, he completely ignored Kaido's challenge and ran away with Luffy. This was an offensive opportunity that Zoro directly let go because he was babysitting Luffy.
How many other attacks has Zoro not taken because he was so devoted to protecting Luffy? How many more opportunities has he let go?
The opportunity cost of protecting Luffy has been Zoro sacrificing several opportunities to attack Mama or Kaido.
Even aside from the counterfactual attacks that Zoro let go, a majority of the other attacks he launched were only launched when protecting Luffy:
- Hiryuu Kaen
- It was launched as a counter to the Boro Breath Kaido used to try and take down Luffy.
- Kokujo Ou Tatsumaki
- It was launched to rescue Luffy from being eaten by Kaido.
The only attack Zoro took independently from his role of protecting Luffy was his initial Rengoku Oni Giri.
Had Zoro not needed to save Luffy those two times, he would probably have never launched those attacks. Zoro has mainly gone on the offensive when it was necessary to rescue Luffy. This reiterates again, that Zoro's top priority has been protecting Luffy.
Zoro's exceptional focus on protecting Luffy seems to have caused him to forego some offensive opportunities. This is the sense in which Zoro was nerfed.