In the high level architecture it says:

The master writes directly to its own local database, which gets synced asynchronously to all the replicas

Yes on this summary page it says:

To ensure strong consistency, Chubby master sends all write requests to the replicas. After receiving acknowledgments from the majority of replicas in the cell, the master sends an acknowledgment to the client who initiated the write.

So in regards to replication, is Chubby sync or async?