A katana really isnt sharper than any other sword afaik, the curve just makes it slightly better for slashing. And you dont slice through plate, this is just insane. "Well made katanas" are no better than the average european sword, theyre just getting close to being on par, japanese pig steel just needs insane work to actually be usable without the weapon becoming complete garbage. They still are though.. That helm couldve been made out of the same garbage aswell, try doing that on an european helmet.
And then theres also the katana tests made now recently, that use katanas made of modern steel, which basically makes them into a new weapon and not a traditional katana.