No, check the link, it's referring to variants of the same product, the ADPS1 (M4575LL/A & M4475LL/B), meaning not all of the first speakers had the subwoofer option built-in.
This model (M6082) has the sub-woofer jack:
http://popcorn.cx/computers/apple/peripherals/audio/powered-speakers/powered-speakers-02.jpg
Curious to see if the non-subwoofer model has the same model number similarly to the ADPS2 white/gray sharing one, and only differentiated by product number. Apple's tech note could also be a typo (not unusual).
Clearly, the sub-woofer model was not enough in demand to offer one, even optionally, on the ADPS2 (M2497 white/gray). If so, your model may be somewhat rare. Or Apple quashed a planned subwoofer project during tight economic times, so no reason to offer the electronics in the new model.