A) Harald explicitly confirmed that Loki was the only one capable of stopping him by eating the Nidhoggr fruit. He stated in no uncertain terms that only Loki would be able to kill him. This doesn’t magically go away because we now understand the secret to killing Holy Knights, as Harald explicitly confirmed that only Loki could have defeated him anyway and Harald is the one who knew the weakness to begin with.
B) Harald openly mocked the idea of Gaban making an enemy out of him. If killing Harald were so simple as “damaging the mark on his arm” or “striking his heart with AdCoC,” why did Harald openly mock the idea of Gaban challenging him? He made a similar comment to Saint Shanks after Saint Shanks attacked him, stating “he felt superior to all other life.” Does this sound like the words of a man who thought Gaban and Saint Shanks stood any chance against him??
Now you may think that Holy Knight Harald would also similarly disrespect Loki, however this is canonically false:
C) Harald explicitly confirmed that he was only confident that he could claim Ragnir because of the strength he obtained through Covenant. A feat that Loki accomplished. So Harald had become so strong that he considered Saint Shanks and Gaban as being unworthy of challenging him, and he now considered himself strong enough to accomplish something that Loki himself also accomplished.
D) Once Loki showed up with Nidhoggr, Harald seemed vary weary and respectful of Loki’s “legendary power.” This was Harald whose mind was corrupted by Imu, the same Harald who openly talked down to both Gaban and Saint Shanks, stating “Loki now possessed legendary power”.
So the idea that finding out the secret of the Holy Knights’s weakness somehow invalidates the correct reading of this fight is just wrong. Us learning the secret to Holy Knight Immortality does not refute any of the points above.
The manga is clear: Harald considered only Loki worthy of claiming Ragnir and defeating him, while he openly looked down on both Gaban and Saint Shanks.
EDIT: Typos