A couple things that I didn't make too clear that may help sort some of these things out:

• patent applications, provisional or not, are published (unless specifically filed to not be, but 95% of applications I looked at were published). But I didn't think inventors would search patent databases before inventing...
• Filing date is the easiest way to establish prior art date, not the only one. If two inventions are similar and filed close in time to each other they could show additional things to establish an earlier date. I forget the specifics but it regularly happened when I would use prior art within a year of the filing date to reject the application that the applicant would come back with notes from meetings or emails or other supporting documents to establish a new prior art date and invalidate my rejection, a product release would qualify, but it has to be before the filing date of the provisional or application where the infringed process is disclosed.

From the limited details I know about the case it looks like apple wanted to use the masimo process, even approached them but then backed out and apple didn't use a different enough process in their watches to avoid infringement.

That seems to me like apple doesn't have a prior art date before masimo for the process used since they approached masimo and the small sections of arguments I've seen from apple are focused on the length of time between application and patent and that at the time masimo was not making consumer devices. Both of which have little to nothing to do with infringement. If their arguments were about the difference in their process vs massimo, or notes or something detailing development before approaching masimo I would have a more favorable eye towards apple.

Again I might be missing something because I have not really read up on this except highlights from articles and a passing interest because of my old job. But was trying to provide some insight to the patent side of it.