that's transmission delays not relativistic effects. The idea of spanner is you have a timestamp of when it was decided to commit. The quorum knows they can't contact each other quickly but they trust each others timestamps and resolve conflicts based on the trusted commit times (which are accurate through hardware). Transmission delays don't undermine the fact there is a very real concept of ordered time in the physical world which is exploitable[1]. Spanner exploits it faster than transmission delays, but with a clock error of 10 ms or something. We have better clocks than that so its probably going to improve...

[1] sans relativity effects which are TINY, and not the limiting factor at the moment.