I don't see how it isn't based on time. They use GPS and atomic clocks to establish the time, establish an uncertainty window, and in Spanner's case will have transactions wait out that uncertainty to guarantee an ordering (globally).
Look up the case where two transactions affecting the same record share a timestamp (or have overlapping error ranges). An non-timestamp tiebreaker determines the order between the two overlapping commits. It is not unlike the pre-agreement on conflict resolution mechanisms for CRDTs.
The context to my first comment included "timestamps within the same window"
