Oh yes they probably did that on their own, yes.
Apple has a huge health team and has clearly done some cutting edge work. Go read their published papers on their Single Lead EKG as an example.
They would rather conceive a medical device knockoff because it would be massively adopted, rather than the top of the line products they were making.
I think that quite a few people would rather make a product that could affect the lives of tens of millions of people, instead of hundreds of thousands of people. Just as their are car designers who want to work on passenger cars and those who want to work on sports cars. This is especially true if they believe that the direction Apple is heading will produce an even better device with very wide adoption.
But you're right, we don't know. But this site is about rumors and speculations right?
You keep making claims (like the SPO2 is "unreliable") that are either factual or not and can easily be proven. If you have evidence for your claim please present it.