I'm not an qualified expert, but I spend a lot of time reading about and using this stuff and I'm extremely skeptical sleep apnea detection is very accurate without the pulse oximeter functionality.

But gosh, I hope this is accurate across the Watch's wide user base.

I wouldn't say it's likely, but I do wonder if the pulse oximeter is being used surreptitiously. In any case, if you could use the pulse oximeter, this functionality would immediately become much more powerful.