This is the description:

The “Compagnie des Gendarmes Ecossois”, better known as the French Scots Guard, are a great example of the term “the enemy of my enemy is my ally” since their shared enmity to England has given the Scots and the French common cause. These Scottish troops are elite heavily armoured longbowmen and form part of the French King's retinue.

Where does it say they are the best archers in the game?

There are more than likely going to be multiple elite archer units in the game.