Is this a bug? (Leviathan vs Tumeken's Shadow)

In the main game the Tumeken's shadow is surprisingly a good weapon against leviathan but I don't know if its some kind of carry over bug from leagues but when you hit him in the spine or from the pathfinder during enrage you hit some... crazy numbers like 100+.

https://preview.redd.it/7qc64wwnloce1.png?width=934&format=png&auto=webp&s=55182663de644cc6ffa9c0b130d49f35ca1e7720

If you want to test it for yourself all you need to do is run in stun and hit him in the spine (its not 100% accurate unlike ranged hitting the spine or pathfinder but if it hits it REALLY hits

https://preview.redd.it/ink9l300moce1.png?width=939&format=png&auto=webp&s=bd578ba245a2467d1af903bf8157b6c676dbccb3

and uh just the dps for fun to show its somehow good (better than bofa unless you have vambs / quiver)

Now I know he's weak to "shadow spells" but this is ridiculous