The square wave is generated by running the sawtooth through an internal voltage divider and comparing it to the control voltage, which gives you an effective range of just over 0 V to just under 4 V (supply/3). The circuit still works, it’s just that you cannot hear a signal that’s 100% on or 100% off 
(this is common in DIY circuits since the alternative is trimpots everywhere, and you need to be able to tweak your CVs anyway so people usually don’t care that much about what happens at the boundaries…)