Sure, but the point is that it did not have enough precision for both a large absolute value, and small time measurements, when based on time since boot.

No, this is not what happened. They converted the 24 bit fixed point value into a different format using different algorithms in different places of the system resulting in slightly different values. The failure was then caused by the difference between these values, i.e. different parts of the system did not agree on what the current time is.