Okay okay I'll admit little suggests anyone else can kill a HK outside of:
a) potentially Joyboy level CoC
b)
perhaps a critical haki mass attack from a PK level fighter
However, since Gaban is less hyping Loki and more so hyping the treasure of Elbaf, this means that Loki's haki had little to do with it. Thus, this means that there's an option c: either Loki's fruit, Ragnir or a combination of both allows him to kill immortals.
Gaban didn't say no one would be able to stop Harald because Harald was stronger than everyone, he said it was
specifically because he'd just keep regenerating:
https://x.com/i/status/2011893582635155780
Somehow, Harald's CoC was preventing his regeneration from slowing.
I don't believe Loki killed Harald because he has Joyboy level haki — Loki genuinely didn't even know he had CoC until 3 minutes ago. It was outright stated that Loki wouldn't be able to kill Harald unless he took the treasures meaning base Loki can't kill a HK either.
I think there's a reason why Xebec believed Loki's fruit to be one of the two keys to stopping Imu, and that killing immortals is the reason. Thus, you have a matchup-specific ability from Loki that doesn't help him from getting locked up by Shanks.