Why not ask Bob?
I'm sure he means the -3dB point. But few speaker manufacturers are honest about the lower end, especially since clean, low bass at high volume requires big, heavy, expensive cabinets and drivers.
Keep in mind that particular rule of thumb applies with a *total* crossover slope of 12dB/octave. Not all crossovers have a 12dB/octave *electrical* slope (24dB/octave is very common in pro gear), and the *acoustic* roll-off contributes as well.
The problem with mains speakers contributing to the low bass is that the mains have to be located best for imaging and the quality of sound above the sub crossover. Those locations are rarely the best place for them to contribute to low bass. Seperate subwoofers can be moved around for the best bass response.
All these things, and more, are just rules of thumb which sometimes have to go out the window to get acceptable bass response. Look up "subwoofer optimization" for some approaches.
Sources/processing: stack of stuff that if it isn't vintage now, it will be soon!
Amps: stacks and stacks of old iron
Main speakers: big DIYSG
Surrounds: Bose graveyard
Subs: a bunch