> Also, there don't seem to be public research papers about it.

The question of atomic clocks aside (whose answer I don't know), Ex-Googlers have created open-source alternatives to Google-internal software in a lot of other cases, too, so I don't think the lack of public research papers could be the reason.

Atomic clocks are not required. That's a misunderstanding of what they're used for.

The atomic clock thing helps improve performance of Spanner, by tightening the SLO of node-to-node and continent-to-continent clock drift.