It says:
The core RMC rules contain 6 weapon skills and 8 martial arts skills, and 1 skill that increases the Defensive Bonus of martial artist professions. Using the rules presented in this section all of these skills are replaced with a single skill called Combat Styles.
So it becomes part of your combat style skill, raising the cost, and you do not have to separately develop the Adrenal Defense skill. That might save you some points as long as you only want to develop one combat style but it will cost you if you want more than one way of fighting.