So, if LJ has this magic power of getting the most out of his guys how come he was able to develop Joey, Nick, & Chase so well, but not Pitmann, Washington, Holmes, or Cooper to the same degree?
Se'Von Pittman was never coached by Larry Johnson; he only played at OSU in 2012 before transferring. Johnson's first year at OSU was 2014.
Adolphus Washington played D-end, 3-tech, and Nose Tackle during his time at OSU, and I think moving between positions affects how good a player can get for obvious reasons. However, he still performed well at each position and had a good career. He was named a 1st-team All-American by Sporting News in 2015, and he got drafted in the 3rd round. Can't complain too much about how he performed.
Jalyn Holmes got most of his playing time in 2016 & 2017 when we had a 4-man rotation at D-end between Holmes, Nick Bosa, Sam Hubbard, and Tyquan Lewis. In that situation, it's tough to rack up crazy stats but Holmes still played well throughout his career and was drafted in the 4th round. I don't think he underperformed at OSU.
As for Jonathon Cooper, you can argue that he has underachieved so far, but injuries have a lot to do with that. He only played in 4 games last season, but he looked good in the little time he got (including against Michigan). I'd like to see him get another season before deciding whether or not he's underperformed.
And, while Gary underachieved under Mattison et. al's watch, UM has had a number of other guys that have overachieved.
Plenty of OSU D-linemen have overachieved under Larry Johnson, as well, which has to be considered. Tyquan Lewis wasn't rated super high as a recruit but he was a 1st-team All-Conference player twice, including winning Big Ten Defensive Lineman of the Year in 2016, and he got drafted in the 2nd round.
Sam Hubbard also wasn't super highly-ranked, and was listed as an Athlete rather than a D-End, but became an All-Conference player who got drafted in the 3rd round.
More recently, Robert Landers and DaVon Hamilton were both 3-star recruits. Landers was a major contributor for 4 years who made a ton of plays. Hamilton was an All-Conference player in 2019, and got drafted in the 3rd round.