"Longitudinally collected HRV metrics from a commonly worn commercial wearable device (Apple Watch) can identify the diagnosis of COVID-19 and COVID-19 related symptoms."
Having first-hand experience, I can confirm that my heart rate was WAY up, and my pulse-ox was WAY down when the virus was at its worst. I used the pulse ox value to make sure I was getting in good breaths (and yes, I know there is a hefty margin of error on these consumer devices.) I use a Garmin, but the sensors are pretty much the same on Apple Watch.
It would be pretty easy for the device to tell I was sick but probably impossible to tell I had COVID-19 instead of the Flu or a bad infection or maybe pneumonia.