I went to Nuki's website to try to find out where you insert a physical key, if needed (Nuki says they supply three), but I found no answers there either. All images I found showed the same slotless center on both the indoor and outdoor knobs, and no key slots anywhere else.
But there's an odd animation about midway down on the home page showing a simplified installation of the indoor knob for Nuki's prior, non-Ultra model, which has a large box hanging down from the knob, and that animation shows a regular door key being first inserted into a key slot on the door's deadbolt before the Nuki's indoor doorknob is placed over it.
I have no idea what to make of any of that.