So from what I can gather this was Ragnar's version of flash freezing someone, I'm not sure if Loki literally means it can't melt or he's just speaking in hyperbole, we did see the skull monster not frozen after ragnar froze it.
Also this was Kuzan's flash freeze and he can also undo his ability like Ragnir.
This means that Loki held back his freeze and has a higher lvl just like Kuzan does.
I think Loki's flash freeze might be more potent regardless, just looking at the difference between Kuzan's ice ball and Ragnar's ice block. It looks more solid visually.
The ice looks darker and more dense. It makes sense why Oda doesn't allow these flash freezes to freeze people to the bone, these attacks come out way too quickly and cover too much ground. The fact that Gunko isn't frozen to the bone tells me this attack can indeed be thawed out but it would require a lot more effort than breaking out of Kuzan's flash freeze.