It's my understanding that Google Spanner is leveraging hardware (direct lines between machines), so the degree at which there is a network partition is different than compared to other systems.
I believe at the end of the day the CAP theorem is too fuzzy for such discussions.
(Fwiw their slo for multi-regional instances is 99.999%, although I have no idea what their measured performance is against that objective.)
"Does this mean that Spanner is a CA system as defined by CAP? The short answer is “no” technically, but “yes” in effect and its users can and do assume CA.
The purist answer is “no” because partitions can happen and in fact have happened at Google, and during (some) partitions, Spanner chooses C and forfeits A. It is technically a CP system."
which I believe is another way to word what I say in my post above.