You get stereo sound. When switch is set to MONO, but both outputs are hooked up, you get stereo. Same with stereo in stereo out obviously.
Again, as long as both outputs are connected, you’re outputting in stereo, not dual mono. If you move the switch to stereo while only hooked up with a TS cable, you’re going to be missing right channel information and the pedal will not sound right. With this incorrect wiring, you’d have to move the Pitch Mix knob to one side to get any semblance of normal operation, but it would be mono and again, you’d be missing the processing of one of the voices.