People in the prison would know if it was a lie or not. So whether you say it or not wouldnt matter. Saying you stabbed 30 people for looking at you wrong isnt going to strike fear into people in the prison ,unless they know you actually stab people.
If this guy was known to hold his own,and people know he only said to be on the documentary and gain privedledges. Who in the prison is going to challenge him for LYING about raping inmates?