Indeed. They admit that it could be done with an traditional relational database but argue that it wouldn’t be able to scale in the same way as with Cloud Spanner. But they are making it sound as though Cloud Spanner has some graph data model.

My interpretation is that they are arguing that they would have to sacrifice the ability to fully model all connections in the system or answer any kind of query (they talk about doing some things “per-user”) because they would have scalability issues or would have to do manual sharding of data. They are considering the full model with all connections as a “graph database”. This terminology is confusing and seems to ascribe extra capabilities to Cloud Spanner.