How is this a Pyhrric Victory?

I've always understood that the rating you got on your victory was based on the amount of casualties you took in the battle, so I don't understand why this isn't decisive or heroic, could someone explain?

https://preview.redd.it/rs9uw4pxej4b1.jpg?width=1920&format=pjpg&auto=webp&v=enabled&s=d6a66f2a888561c73a0cba3408fb3ebd02190260