Tidus may know the sign of the Blitzball hand thingy, but that doesn't mean he is good at it. He was probably one of those arrogant athletes that got high off his own fame.

"HA HA HA HA HA! I'M THE STAR OF THE ZANARKAND ABRAHAM LINCOLNS!! I DON'T NEED TO LEARN THE GANG SIGN!! HA HA HA HA HA HA!!!"