Yes, I believe so. Since in Postgres, snapshot isolation (on top of which its serializable implementation is built) provides a stable view of the system at a point in time, and also doesn't allow stale reads, transactions _can be_ strictly serializable.

However, if multiple synchronous standby replicas are configured, then this means that stale reads are possible, since Postgres by default does not necessarily wait for all replicas to ack the write. It depends on the replication configuration and where reads are directed.