What's the difference between strong consistency and sequential consistency?
In the case of the Spanner doc: it appears to be more strict than linearizability due to transactions. The difference between linearizability and sequential consistency is that latter doesn't need to preserve the real time ordering of requests.
https://jepsen.io/consistency/models/linearizable
The combination of linearizability and transactions is called strict serializability:
https://jepsen.io/consistency/models/strict-serializable
I'm not sure if spanner is supporting strict serializability.