Harald didn’t even know that immortals exist until earlier that morning. He was more so banking on the fact that the fruit has a world-ending level of power, and that it would be somehow enough.
The issue I also have with this is that base Loki killed Harald while he was in the process of fully regenerating, so the fruit didn’t even cancel the regen in the first place. In the present day, Gaban and Loki also confirmed that HKs aren’t truly immortals while implying that aCoC is the secret sauce to killing immortals.
My headcanon is that the fruit just gave Loki such overshelming power, and if you recall Rayleigh’s words about overall strength and CoC strength then everything fits.
My issue with this theory is that, no one's haki has ever immediately jumped tiers due to eating a fruit.
Sabo's haki didn't get any stronger after eating the Mera. BB's haki didn't get any stronger after eating two top-tier fruits. When Luffy ate the gomu gomu, albeit being a kid, he still didn't awaken CoC until he was 17. And when his fruit awakened as a legendary zoan, his haki itself remained the same strength as Luffy still can't kill immortals yet.
It's manga fact that base Loki could not kill a HK, so we
know that base Loki's ACoC is not any different to say Gaban's in terms of Loki's being some special new level. Plus, Gaban being a haki scholar, if it was about haki I'm sure he would've said so. Instead, he said it was about the fruit.
Harald realised he was immortal and was wondering what he could do to die
before he asked Loki to eat the fruit.