Earlier quoted context omitted.
The atomic clock is the key to enable distributed transactions and Google has a proprietary lock on their atomic clocks for Spanner.
Do you know what is exactly proprietary in spanner? AFAIK most (all?) of the ideas existed before in the theoretical clock synchronization literature.
So while the spanner paper is open to be implemented by any vendor, they don't have the proprietary advantage that Google has - the atomic clock. So Yugabyte, CockroachDB don't rely on atomic clocks. I tried to get to the ground level basics of this, but I haven't understood this matter completely yet.