Assuming Ragnir is the one who put the Galley-La on ice and only ragnir can undo their freeze, it just means:
1) It's genuine hax but only activates when you get hit clean — otherwise, Loki would've frozen Imu by now.
Hell, Loki hit Imu clean and didn't decide to freeze him — if it was such a sure-kill move, why didn't Loki freeze Imu then and there? Could chalk it up to PIS perhaps
2) Imu didn't really try very hard to break the ice, he just sent a random nameless explosion to it. What if someone coated in strong haki or with their own hax was inside, could that work?
3) If it is a one-shot kill, we both know that it wouldn't be as easy as that in any actual 1v1 between Loki/Ragnir and a top-tier. Sure, if it lands it could be a one-shot kill, but all it'd mean is the top-tier will have to dodge or block Ragnir and avoid getting hit clean. It's like Akainu's Meigou, Crocodile's moisture drain, Magellan's poison etc... It's just another ability you have to play around as opposed to proof that Loki actually neg-diffs top-tiers.
Take Kizaru for example — he'd run circles around Loki so good luck trapping Kizaru in ice.