If everyone is synced to +/- 20ns, that's great. Then when someone flies over your datacenter with an GPS jammer (purposeful or accidental), this needs to not be a bad day where suddenly database transactions happen out of order, or you have an outage.

The other benefit of building in this uncertainty to the underlying software design is you don't have to have your entire infrastructure on the same hardware stack. If you have one datacenter that's 20yrs old, has no GPS infrastructure, and operates purely on NTP - this can still run the same software, just much more slowly. You might even keep some of this around for testing - and now you have ongoing data showing what will happen to your distributed system if GPS were to go away in a chunk of the world for some sustained period of time.

And in a brighter future, if we're able to synchronize everyone's clocks to +/- 1ns, the intervals just get smaller and we see improved performance without having to rethink the entire design.

TrueTime is the software algorithm for managing the timestamps. It’s agnostic to the accuracy of the underlying time source. If it was inaccurate then you get looser bounds and as you note higher latency. Google already does everything you suggest for TrueTime while also having atomic clocks in places.