This is a passive skill, basically anyone that can and does equip a shield, will fly up the achievements because it's currently being counted as a separate class
Example:
Joe and Bob are new to the game and decide to level up together
Joe chooses gladiator and gets a shield right away
Bob decides to play archer
Both Joe and Bob level at the exact same rate as they always play together. When both reach 50 Joe has 2x the amount of points on ffxivpro, as Bob does.
Secondary classes are just that, please stop counting them as a separate class.