I think you're right, and now I understand why the rules seemed buggy to you but not to me. You're considering persisted messages that need to be compatible with many versions of the schema, whereas the discussion and rules are formulated in the context of RPC messages between services which only need to be compatible with at most three versions of the schema: the version that generated the message, the version before that, and the version after. The README could do better to clarify that.
In the persisted messages scenario, there is one change to the rules: you can never introduce a required field (since old messages might not have it). Not even asymmetric fields can be promoted to required in that scenario.
In the persisted messages scenario, there is one change to the rules: you can never introduce a required field (since old messages might not have it). Not even asymmetric fields can be promoted to required in that scenario.