That's correct. The over lapping came to the point of not allow enough recovery time for the upper body. Though the body always acts as a unit, nothing is independent really. The CNS, when called upon, can affect all of these units. The key to progression progress in lifting is the CNS. I know some think that over training is unlikely and almost impossible...than that's their belief. I hold completely opposite views (for whatever that's worth).
If thinking that your only working chest on chest day, you are mistaken, sorry to say. To a lesser degree even the lower back & abs are involve. And of course, to a greater degree, the triceps and all three heads of the delts, lats and even traps.
Now 6 weeks, without any activity, for the body to begin the process of reverting back to it's natural body type, might be a little longer than most accept. But I can't disagree with Mentzer, he had his own views. Anyone who has ever been in a arm or leg cast can give witness to the absolute shrinkage of that body part, in a some what short period of time..
I've had good luck with the following training idea, which can allow a little longer rest period between muscle groups than normal. Your keeping the workout short and to the point. Not too many sets and not too many exercises. Point of fact, people will probably make faster progress if they cut their workout down to at least 1/4 to a 1/2 of the movements performed. Instead of 18-20 sets per body part, try cutting that down to 9-10 sets per body part max.
Monday
Upper body
Wednsday
lower body
Friday
Upper body
The following Monday
lower body
Wednsday
upper body
Friday
lower body
Alternate the next workout, starting with upper body again..etc, etc,etc...
Good Luck.