Imunko couldn't break the ice with all the haki in the world. The fact the ice doesn't melt means it's not actual ice....think of it like a hax ability that traps you in but it happens to take the form of ice. It's just magic.
This lvl of hax is not uncommon in other verses. We're just getting introduced to god tier abilities so I wouldn't be surprised in the least.
Actually a good way to look at this is: the way Kuzan's ice treats commanders is how Ragnirs ice can treat top tiers. We've just scaled up.