I don't have an issue looking at divisional records since you have to win your division in order to play in the championship game and 75% of your conference games come against the division.
Only once in the time period specified was the team to play in the Championship game not the team that won the most (or tied for won most) games in the conference - that was 2013 when VT was 5-1 in divisional games but lost out to Duke for the Championship game (who went 4-2 in the division) as Duke won its 2 cross divisional games and VT lost theirs.
GT is pretty much going 1-1 across division. it is 6-0 over this time period (2013-2018 when Pitt joined Coastal) against non Clemson Atlantic teams (With FSU being its lone ACC win in 2015) and 1-5 against Clemson during that span.
GT is a pretty ridiculous 13-1 against ACC teams they don't face every year under Johnson - which certainly lends some credence to the idea if you don't see it often Johnson's offense and program is really hard to beat.
Miami has been pretty unlucky (partially because they play FSU every year). They have not finished under .500 in division record over the last 6 years and twice been 5-1 and twice 4-2, yet only made the Championship Game once.
In terms of division games GT is 10-14 the last 4 years (which is 5th out of 7 teams, Mia has 17 wins, VT and Pitt 14 wins, UNC 12),
14-16 over 5 years (5th out of 7 teams, Miami has 20 wins, VT 17, UNC and Pitt 16), and
18-18 over 6 years (T3, Mia 24 wins, VT 22, GT, Pitt and UNC all have 18).
GT has one of only 2 winless division years, UVA has the other in 2013, and UNC has the only 6-0 year in division, in 2015 - same year GT went 0-6.
FWIW, the Division winners went 6-0 (UNC 2015), 5-1 (Miami 2017, VT 2016) and 4-2 (Pitt 2018, GT 2014 and Duke 2013).
Johnson had a good run of 11 years here, but it was certainly slightly weighted toward the earlier years - that should be expected given the ACC got stronger during his time in general and Clemson became a powerhouse.
But it is also fair to say he wasn't winning games consistently enough over the last 4-6 years in the division to be a consistent threat to win the Coastal.