This is an unfortunate report in a lot of ways. First, the title is incomplete. Second, there’s no context as to the purpose of the test and very little about the parameters of the test. It makes no comparison to other PostgreSQL architectures except one reference at the end to a standalone system. Third, it characterizes the transaction isolation of this system as if it were a failure (see comments in this thread assuming this is a bug or a missing feature of Postgres). Finally, it never compares the promises made by the product vendors to the reality. Does AWS or Postgres promise perfect snapshot isolation?
I understand the mission of the Jepsen project but presenting results in this format is misleading and will only sow confusion.
Transaction isolation involves a ton of tradeoffs, and the tradeoffs chosen here may be fine for most use cases. The issues can be easily avoided by doing any critical transactional work against the primary read-write node only, which would be the only typical way in which transactional work would be done against a Postgres cluster of this sort.