He's not saying that it doesn't exist, he's saying he doesn't believe that's who startups should hire.
He's making the argument that 10x isn't solely about the person and their technical abilities. You could have a "10x developer" who joins a team and performs poorly because they're not motivated by what the company is building, or they have a shitty boss. What makes the person a good performer is the environment they're put in combined with their technical abilities.