That is most likely true from November '72 on. From August '72 to November '72 (which is all '73 model year production) it's not a guarantee, the TSB I posted above lists the final crossover date of November 1, 1972.
My guess is up until November '72 there was probably some SBJ stuff going out as previous stock was used up, as we all know Ma Mopar had a tendency to do when production changes were made. So the closer to the TSB date the less likely anything would have left with SBJ. Show me a 9" drum spindle that takes a large upper ball joint and I might consider changing my mind, but I'd say that everything after November 1 '72 is large ball joint regardless of disk/drum brakes, but prior to that a drum car might still have a chance of being SBJ.
Early '73 I'd put into the "who knows" category. If you weren't the original owner who knows what happened before you got it. But for that matter, who knows what happened at the factory before the TSB documentation took effect either. After the first few months of production for the '73 model year though I'd say '73+ would probably be LBJ from the factory at least.