Is this 90% a upper or lower bound? I mean did they try to measure luck and found that it accounted for 90% of returns?(lower bound because their measure of lu

Is this 90% a upper or lower bound? I mean did they try to measure luck and found that it accounted for 90% of returns?(lower bound because their measure of luck was probably imperfect) Or did they measure talent and were able to account for 10% of the returns(upper bound).