I'm not a ZKK fan at all honestly.
But I can definitely see the connection for the theory.
The black blade can work too true but must happen close to people.
If Oda really wants Zoro to be seen as the next sword God or next Ryuma Wano is the only place after this it's impossible to build that.
how many chapter do you think do we have in Onigashima before the people can see the floating island?
I bet around 15, that enough time for Oda to elaborate easy narrative to push Zoro for the last thing.
I am 100& sure that Oda didnt build up all the sunachi thing with Zoro (Oda introduce sunnachi as first with Zoro, Zoro had a flashback as a kid with the sunacchi) then used Sunachi with the samurai raid that failed then used Sunachi with the 9 scabban when they push Kaido down from the balcony inside the dome, the only character that didnt use sunacchi in Wano is Zoro, hyronic right?
Oda introduce sunacchi only due to Zoro, but hasnt use this concept directly with Zoro.
You dont need an elon musk to understand that there is more boiling for Zoro, Oda is waiting for the right moment.