I don't get it, it's specified anywhere that any meitou are unbreakable or undestroyable ? Only if it becomes black (trough haki or black blade right ?) mihawk said that, so sandai being broken by that weird sword (which by research is a sword breaker) of King is normal since he can break it the moment he clashes with any zoro's swords which are not infused of haki or not in zoro's hands. We saw a navy guy destroying yubashiri because of his power when zoro was holding yubashiri.
But if King breaks Zoro's sandai even when it infused with zoro's haki, that's another thing and crazy and yes really hype King.
At the end Zoro will have black blades which are super rare.