I've watched him a couple times, and his knuckleball is insane. He changes speeds with it (66-81) and it actually looks like he can somehow command it. He'll also sneak 85 MPH fastballs by people from time to time.
i think it's some idiot making a superficial connection between his domination of AAA pitching and his struggles in the majors in 150 PAs at the age of 21/22. it's lazy and it's why a lot of scouts just suck balls. Brett Jackson hit a HR off Shelby Miller this year and Rizzo didn't. Rizzo is the suck.