If Killer's attack can pierce through Zoro, there's no reason to assume Katakuri's Mogura can't do it too.
Sure, Mogura can pierce through Zoro if it lands cleanly. It could never land on Base Luffy even pre FS, and Zoro can block with his swords.
Hell, Zoro can cut through Mogura, and Kata has never demonstrated the ability to harden his spear.
Kata looks far enough into the future that he knows Zoro is going to use an attack before Zoro even thinks about using it.
It's a matter a disrupting your opponent's rhythm.
In a fight, if you can fight at your rhythm, you can't fight properly. Kata's ability is one that you either counter it with a FS of your own, or you simply can't go all out.
Even if Zoro tries to simply "counter" Kata's hits as they come, Kata will know about it.
I doubt Kata can shut down Zoro like that. I mean, Kata couldn't even shut down G3 Luffy like that, and Zoro's attacks execute much faster. I'm not going to assume Kata can prevent Zoro from attacking, when he couldn't do the same to G3 Luffy.
Then how do you justify the gap between Zoro and Onigashima’s horn being 6 kilometers when those tiny rocks that are visible from the top of the skull not even being close to 1KM? You could probably fit like 10 of those rocks max between Zoro and the horn lol.
I've given you the scaling, but I've explained why I think the size in that panel would be unrepresentative. After Wano is over, we could look at more consistent depiction of the size of Onigashima and not just cherry picked panels. Alternatively, Oda might give an official size.