I have a biofeedback hobby and have done experiments with HRV. R-R intervals per pulse are quite useful. 5s reporting intervals are too long. Of course I use a EKG Polar Monitor although the literature seems to think a quality PPG or even an advanced radar-based system ought to perform well enough.
Annoying omission that only the latest "4th gen" heart rate sensor was tested, 2nd gen (Apple Watch <6, all SE models) and 3rd gen (Apple Watch 6-11) are not mentioned and it would have been very interesting to compare.