I think you are vastly overestimating how good the CAR backup QBs were. Kyle Allen and Mason Rudolph look pretty comparable on paper as the primary QBs for the year, both 62% completion, Allen with more interceptions per game and slightly less TD per game compared to Rudolph.
Going by passer rating from
https://www.pro-football-reference.com/years/2019/passing.htm:
PIT:
- Roethlisberger - 66.0 (2 games)
- Mason Rudolph - 82.0 (8 games)
- Devlin Hodges - 71.4 (6 games)
CAR:
- Cam Newton - 71.0 (2 games)
- Kyle Allen - 80.0 (12 games)
- Will Grier - 33.2 (2 games)
Average passer rating per game:
- PIT - 76.025
- CAR - 73.025
I don't think passer rating is the be all end all of QB stats, but it is there for quick comparison. They are not substantially different, so I am not sure what you saw from Carolina that makes it look that much better. This does not factor in injuries the various players had etc.