[AdamN]

This time accuracy would need to propagate to all their hosts, not just the ones in a single DC. I presume they have hosts in the EU, London, NYC/NJ, Tokyo, Chicago, etc... I imagine 100ns accuracy with that kind of global installation diversity isn't straightforward.

[pclmulqdq]

The gold standard for PTP is to use separate GPS-disciplined atomic clocks in your separate points of presence, and PTP within each one. These are about $10k each, so it is not that expensive even if you have 100 datacenters.

[namibj]

Yes it is, GPS is 1foot ~ 1ns; thus a few ns are trivial with a vaguely decent receiver.