This is a very interesting story and I imagine there is some amount of ML used to gradually improve the accuracy of results for a specific user.
Unrelated: It is called the Apple Watch. :)
Unrelated: It is called the Apple Watch. :)
There are tons of people that test these devices against chest straps online, and almost all of these bands give results that are surprisingly accurate.. or at very least about what you'd expect out of a wrist based monitor.
ML only works for these types of measurements when you have some kind of reliable source of truth or fitness function to compare against.