This paper was partly funded by Polar which Rob cited several times and used for his Polar H10 chest strap:
https://pubmed.ncbi.nlm.nih.gov/31004219
RR interval signal quality of a heart rate monitor and an ECG Holter at rest and during exercise
And the paper mentioned here about the Polar OH1 was devalued by Polar:
Polar supports accuracy validaton papers about the Polar H10, and devalues accuracy validation papers about the Polar OH1. And the only one who seem to care about scientifically validating smartwatches seem to be Rob ter Horst: