I think you're mixing up the D problems with the wrong coaches. Coombs was gone the 1 of the 2 years the D was absolutely horrible. Look at the INT and pick 6 numbers under Coombs:
Not coincidentally, Coombs also helped shape some of the Ohio State’s best defensive units. From 2012 through 2017, Ohio State was second nationally with 101 interceptions and first with 17 that were returned for touchdowns.
Can't catch it if you aren't looking for it.