I'm not smart enough to know how to argue this but I certainly didn't get the impression that pitchers were letting up on anyone. Sure there were weak spots in lineups but that's still true today. Using your let up theory pitchers today could certainly let up on Perez and Rogers.
Looking at middle of the pack teams from 68 they seem similar to today. The Twins had Carew, Killebrew, Oliva, Allison, Tovar - the Pirates had Stargell, Clemente, Clendenon, Alou.....both essentially .500 teams. They had some weak spots in their lineups but certainly batters no one wanted to face.