I don't know about all hyperscalers, but I have knowledge of one of them that has a large enough fleet of atomic frequency standards to warrant dedicated engineering. Several dozen frequency standards at least, possibly low hundreds. Definitely not one per machine, but also not just one per datacenter.

As you say, the goal is to keep the system clocks on the server fleet tightly aligned, to enable things like TrueTime. But also to have sufficient redundancy and long enough holdover in the absence of GNSS (usually due to hardware or firmware failure on the GNSS receivers) that the likelihood of violating the SLA on global time uncertainty is vanishingly small.

The "global" part is what pushes towards having higher end frequency standards, they want to be able to freewheel for O(days) while maintaining low global uncertainty. Drifting a little from external timescales in that scenario is fine, as long as all their machines drift together as an ensemble.

The deployment I know of was originally rubidium frequency standards disciplined by GNSS, but later that got upgraded to cesium standards to increase accuracy and holdover performance. Likely using an "industrial grade" cesium standard that's fairly readily available, very good but not in the same league as the stuff NIST operates.