Yes, the Apple Watch is what provides the sleep stage data. If you only have an iPhone there's no in-depth sleep stage data which is what the model uses to predict a sleep quality score. The disclaimer didn't show in the video since I had closed it already.
Do you happen to know if the Oura ring, the Whoop, or other biometric devices can feed in that data to Apple health? If so you should make people aware of that possibility.
It currently only supports Apple Watch, but I'll release an update soon so it can support Apple Health compatible devices. The only requirement is that those devices can export sleep stage data and other metrics to Apple Health, thanks for the feedback!