When I say it's not because of Harald's strength, I mean if you take HK Harald's regen away and just have his strength buff, he canonically loses to Gaban and young Shanks lool.
Nothing suggests HK Harald without regen is stronger than someone like Prime WB.
The problem is that this is a high top-tier who instantly heals from every attack. Atp there is nothing anyone can do, hence Gaban's statement. At some point, you'll get tired and he'll kill you.
Harald is not like DR Xebec — Xebec has a CoC barrier which was the issue. Harald did not have a CoC barrier, he just healed from everything.