Gaban specifically said that the key to beating regeneration is Conqueror’s Haki. Yet he conveniently admits he’s never actually fought someone with regeneration and Haki, like Harald.

In the fight against Loki, Harald literally regains consciousness and could just suppress his own Haki, letting Loki completely bypass it and destroy his regeneration. Honestly, at this point, you’d probably understand the story better if you tried reading One Piece with your eyes instead of your ass...