That is less specific than, and in contradiction to, the uo.com wiki.
Here's what the UO wiki says: "Mystic Weapon – The mystic converts mysticism skill into an equipped weapon’s required skill for a penalty to mysticism skill based on mastery level"
Based on the literal meaning of the official wiki that would lead me to believe that if I have 120 mystic, and enable the mastery while holding a sword, I would end up with 0 mystic and 110 or something swords.
What you suggest is that activating the mastery behaves more like the mage weapon property.
These are two vastly different meanings. In particular, one lets the mystic master still use a RC and the other does not.
Can you confirm which is the correct interpretation, and perhaps what the penalty is?