Monks have a professional ability that increases the size of the Martial Arts attacks from small to medium. The intention has always been that this was for unarmed attacks only.
Weapon Kata is a Martial Arts Combat Style that allows a specific weapon to be used with the Martial Arts, and thus is based on the Martial Arts skill, not a separate weapon skill.
Weapon Kata also allows a character to use Chi Defense, and Monk Attacks (multiple attacks, and multiple foes - from the sidebar next to the profession description) with a kata weapon.
The professional ability does NOT apply to weapons wielded through a Weapon Kata.