I think we're conflating Snapshot Isolation and MVCC (e.g., Snapshot-isolation/MVCC). MVCC is a general concurrency control mechanism, not an isolation level. Coupling MVCC and Snapshot Isolation is like saying "using locks provides serializability," which is not true in general--it depends on how you use the locks.

That said, Snapshot Isolation can lead to anomalies like Example 1.2: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.142... If you perform additional synchronization (e.g., 2PL, as you describe), then you can get serializability.