The Soup'er does NOT provide any DC isolation on the speaker lines.
So you still need to make sure that neither audio output lead has a DC path to ground, as you saw in the Appendix of the manual.
If you have one of the newest, rev A(c), circuit boards, there are some extra pads provided (JP2 and JP3) where the appropriate blocking capacitor(s) can be mounted. Then you cut the exposed trace(s) between the pad(s) on the bottom side of the board to put the cap(s) into the circuit.
A powered speaker with a self-contained battery supply can be used without needing a DC block. But if either of the external speaker connections has a DC path to the same ground as the BJ (like back through a wall wart power supply) it would short out and damage the audio chip.