Actually, your video shows exactly that: it was Sexton's defense, or, rather, the lack of it, that allowed Young to make a game-winning shot. Young had only 4 seconds to make it and Sexton has not even tried to slow him down. And he did not have an excuse of being too short: Young is shorter and Sexton is probably faster than him.
That is the main knock on Sexton: there are quite a few players who can generate offense on his level but Collin is literally one of the worst defenders in the NBA among the vets.