That sixteen optical receptors in a circle measure unreliably or "wrong" is extremely unlikely. Judging multiple reviews where the reviewers compared the Watch4 and the workout tracking to one of those chest belts, these optical sensors nowadays are measuring incredibly accurate (at least when they are as big as the ones on Galaxy Watches newer than the Galaxy Watch, but especially the circular ones since the Active2).
I would actually assume that it's a real heart rate reading. You can, of course, verify it by sensing an artery (the carotid artery can usually be felt very easily) and count manually for a certain amount of time to see if it actually matches up. The values on mine actually fit perfectly.
