A lot of folks are saying that Masimo is not a patent troll.
Technically this is correct. However, their behavior is controversial.
The Masimo patents the ITC found Apple infringed were not filed until 2020, well after pulse oximetry was added to the Apple Watch.
Masimo engaged in what is called "continuation practice" where they continuously filed patent applications relating back to the original 2008 applications and altering the claims in the patents to circle around the Apple Watch. In other words, Masimo was targeting Apple with new patents after the fact. While this is perfectly legal, it is also controversial. Recently a federal judge slapped down Sonos for doing a similar thing in their patent tangle with Google (although that ruling is likely to be reversed after appeal).
Also, Masimo's smart watch was likely developed only to satisfy the "domestic industry" requirement needed to get an import exclusion order (the Masimo watch is unlike most of the medical grade products they produce, so it is strange that they would get into the consumer smart watch business all of a sudden).
In an ideal world, Masimo's earlier patents would have prevented Apple from adding the infringing tech to the Apple Watch, but they didn't. Apple has not been found to infringe any of the earlier Masimo patents, only the later ones that were crafted specifically to target Apple after the fact. Again, perfectly legal, but controversial.
As an aside, after having reviewed the infringed patent claims, I believe Apple can design a workaround by moving the pulse oximetry calculations to the iPhone and only using the watch sensor to collect raw data and send it over to the phone. This is because the Masimo patent requires the calculation to be performed by the watch. If Apple did this, it would force Masimo to file yet another patent application (if that is still an option, not sure it is any longer) to try and stop that from happening. But I doubt Apple will try this.