The mentioned study says only something about the accuracy of the algorithms of the Polar H10, and says nothing about the accuracy of the Polar H10 against a high quality motion artifact free ECG device.
The Polar H10 was not tested against a high quality motion artifact free ECG device:
in the study mentioned above.
Two other studies use high quality ECG devices as reference device:
One study says, that a Polar H7 chest strap is the closest to an ECG device used in clinical research, followed by an Apple watch:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6732081/
A second study using a g.Nautilus references devices says, that the Polar OH1 can be used instead of an ECG device as far as accuracy is concerned:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0217288