You made a mistake with your mix node. Your material node setup says “I have a hair material, I have a gray to black intercept dependant transparent node, and I mix them fifty-fifty” when he should say “I have a hair material, I have a transparent node, and I mix them with a gray to black intercept dependant factor”
On the left : What you want -> The Hair info input affects the Mix factor of the Mix node
On the right : What you do -> The Hair info input affects the color of the Transparency node, but in fact your hair is entirely transparent at a value of 0.5