So what? The attack doesn't materialise inside the body. It starts from outside, bypasses the bodies defence and then attacks the inside. Doflamingo knows that, Vergo knew that. The scan literally shows us Vergo was going to counter his Devil fruit powers with CoA but it didn't work, why? cause laws haki was stronger as he himself says straight after cutting him.
Haki counters devil fruit powers. Devil fruit powers triumph over Haki if your haki is weaker lol.
Flash forward to Kaido, Law could not separate Kaido and Big Mom, why? Cause he literally says their Haki (he did not say CoO or CoA or CoC or any of their advanced forms, just Haki in general) was stronger. According to you, Kaido covers his body with AdCoC, so you're literally saying his Haki is stronger thaKaido'sdo, which is nonsense. He only covers his club with it as the Manga shows us.