Data Centers don't lose network connectivity - designed to have ingress and egress far enough apart that a local disaster can't take down the entire ring. Also, as I noted, one of the first things that enterprising sysadmins do is toss a Stratum-0 receiver (GPS receiver) on the roof, which feeds into the NTP protocol.

Finally - the entire purpose behind TrueTimeAPI is to sync up spanner's commits so they are consistent across the replicated/sharded nodes. Without Network, that database capability would come to a halt way before network time became a problem.

The bigger issue is the <10ms requirements. NTP over the Internet does not get you that consistently, and certainly not at the 99.9% success that Google was able to achieve with TrueTime API.