Can't access that website from work, but from the name it looks as though it is about knives, not swords. Big differance. Regardless, Nitrogen has no effect on the hardness of steel. If you quench (reduce the temperature of white or red hot steel rapidly) into liquid nitrogen, you will shatter the blade, probably quite explosively. You CAN cryonically treat a blade using nitrogen (or any other inert supercooled gas), but the quality advantage of the end results are debated. You must remember that too had equates to brittle.