I decided to not trust the documentation and, as of version 0.9.5697 of datomic-free, :db.cardinality/many attributes can be :db.unique/identity, in disagreement with:
Uniqueness can be declared on attributes of any value type, including
references (:db.type/ref). Only (:db.cardinality/one) attributes can
In testing I found that the schema of a :db.cardinality/many :db.unique/identity attribute was accepted and that entity upsert worked fine across multiple instances of the attribute (i.e. the right behavior).
How can we get the documentation updated?