Dude Yuji beat the crap out of mahito

False equivalency. Mahito was vastly handicapped against Yuji. Can't IT him, can't DE him, Yuji attacking his soul and whatnot. These limitations doesn't apply against Hakari or Kashimo.

Even nanami could protect his sould with Cursed energy.

Junpei arc Mahito and Shibuya Mahito have too much power gap. The one against Nanami used CE to protect himself didn't even have a domain lol

kashimo is a cursed object just like sukuna so he must have knowledge of soul.

Not even a bit. Kashimo's soul has fully taken over that human body. The og human's soul isn't there anymore. Yuji on the other hand houses two souls due to being a "cage" instead of a simple vessel.