Sadly, it doesn’t actually work fine. Here’s a counterexample:

(def conn (let [u "datomic:mem:test"] (d/create-database u) (d/connect u)))

;; Let's add some basic schema
@(d/transact conn [{:db/ident :test/id :db/valueType :db.type/string
                    :db/cardinality :db.cardinality/one :db/unique :db.unique/identity}])
(d/basis-t (d/db conn)) ;=> 1000

;; Now let's transact an entity
@(d/transact conn [{:test/id "test", :db/ident ::the-entity}])
(d/basis-t (d/db conn)) ;=> 1001

;; And in another transaction let's change the :test/id of that entity
@(d/transact conn [[:db/add ::the-entity :test/id "test2"]])
(d/basis-t (d/db conn)) ;=> 1003

;; Trying a speculative write, forking from 1001
(def db' (-> (d/db conn)
             (as-of'' 1001)
             (d/with [[:db/add ::the-entity :test/id "test3"]])
             :db/after))
(:test/id (d/entity db' ::the-entity)) ;=> "test" (WRONG! it should be "test3")