I don't buy that logic. Galante was dead before Bonanno wrote his book, so nothing he said could have hurt him. He also talked openly about several other individuals he was friends with who were deceased at that point (Maranzano, Mangano, Profaci, Magliocco...) and didn't shy away from describing the roles they held in the hierarchy of their families.

With what we know about Garofalo and Morales holding the position, I don't believe Galante ever served as Bonanno's underboss. I think it's a similar situation to the misconception that Sonny Franzese was the underboss for Colombo in the 1960s when we know Colombo's underboss was actually Charlie Mineo at that time. (Or the idea that Bonanno was Maranzano's underboss when we know it was actually Angelo Caruso).