First to grant your point, the Reds interleague schedule this year was the hardest by a hair.
The Reds opponents are a combined .5282 winning %, the Brewers interleague opponents are a combined .5278, the Pirates opponents are a combined .517, and the Cardinals opponents were a soft .4593.
I'll admit, that seems like a huge gap. And if over the course of the season, the Cardinals opponents were that much softer than the Reds opponents, it would result in a significant advantage. However, over the course of 15 games, a team with a .459 winning % would expect to win 7 games. Over that same stretch, the .528 winning % equates to 8 games.
The Reds went 6-12 against AL teams, the Brewers went 6-9, the Pirates and
Cards went 8-7.
Really the Reds and Brewers have no one to blame but themselves.