This is entirely wrong. Zoro himself stated with future accomplishment, not his own, meaning it´s something he would essentially become and gives up now for the current Luffy.
And once again, Kuma underlines Luffy´s head for the WG would be enough, Zoro´s never was.
Kuma never did that, he acknowledged hurting Luffy after Zoro´s speech would be dishonorable.
Luffy´s head
Zoro himself admits his head is not enough, hence argues with being future WSS. Also admits it´s a loss for Kuma, only that since he will become WSS someday, it´s not too much of a loss, still a loss regardless.
Both Zoro and Kuma say Kuma not killing Luffy after this is a matter of honor, not replacement of Zoro. If Zoro´s was enough, there would be no meaning to torture him, just kill him and present his head to WG.
Later on Shabondy Zoro himself states he survived due to Kuma´s mercy, meaning he knew Kuma never meant to kill him.