Yeah, this basically boils down to "a potential pitfall, but consistent with documentation, and working as designed". Whether this actually matters depends on whether users are writing transaction functions which are intended to preserve some invariant, but would only do so if executed sequentially, rather than concurrently.

Datomic's position (and Datomic, please chime in here!) is that users simply do not write transaction functions like this very often. This is defensible: the docs did explicitly state that transaction functions observe the start-of-transaction state, not one another! On the other hand, there was also language in the docs that suggested transaction functions could be used to preserve invariants: "[txn fns] can atomically analyze and transform database values. You can use them to ensure atomic read-modify-update processing, and integrity constraints...". That language, combined with the fact that basically every other Serializable DB uses sequential intra-transaction semantics, is why I devoted so much attention to this issue in the report.

It's a complex question and I don't have a clear-cut answer! I'd love to hear what the general DB community and Datomic users in particular make of these semantics.