I suspect the fingerprinting aspect is more indirect: mach_absolute_time is the most accurate way to measure small differences, so if you're trying to measure subtle differences in performance between different devices on some specific task, mach_absolute_time would be the way to go.
If the service can learn the individual but current values of mach_absolute_time, then after a minimum of two requests you can likely compute N and distinguish between each device making a request.
This is possible because devices never reboot at exactly the same time.
And there’s the problem. Different devices have different uptimes. If you can get not only the uptime but a very very accurate version, you’ve got a very strong fingerprint.
https://github.com/bwbuchanan/absolute_time/blob/master/ext/...
https://rubygems.org/gems/absolute_time
Yes you want this in a development environment or perhaps in some sort of macOS app for developers, but probably not in most apps without a very specific and limited need.
You can fingerprint a device using this because you know the wall clock difference and you know the previous cpu cycles. So you can assume any device with appropriately more cpu cycles may be the same device.
We’re talking measurements taken from different apps using Google or Facebook sdk.