mach_absolute_time returns time in ticks, there's a separate API that returns the resolution of this clock (which is already used).

The manpage explicitly says that mach_absolute_time and CLOCK_UPTIME_RAW are the same clock:

     CLOCK_UPTIME_RAW   clock that increments monotonically, in the same manner as CLOCK_MONOTONIC_RAW, but that does
                        not increment while the system is asleep.  The returned value is identical to the result of
                        mach_absolute_time() after the appropriate mach_timebase conversion is applied.

Switching from mach_absolute_time to CLOCK_UPTIME_RAW would therefore bring us no improvements, and would complicate the code base because clock_gettime is only available starting from macOS 10.12.