There are different sets of characters associated with Luffy and Zoro.
Luffy's fight has been about Joyboy/Nika.
Zoro is about Ryuma/Oden.
Luffy saves the day in Onigashima, while Zoro does it in Wano.
They're different sets of characters waiting to be saved in two different settings.
Also, Zoro dismembered King. If Oda wanted to have Zoro kill King (this is unnecessary, and we know how Oda feels about death), he could have done it and it would been believable.
He also managed to leave a scar before being aware of the power he could use.
How Zoro gets back up is up to Oda.
He chose bullshit for the first time around, it's not hard for him to manage another way.