The point is on the ownership of the database and the trust among participants.
In Datomic there is a single owner that has full control on the data (probably you, or a company that you fully trust).
In the blockchain use-cases there are multiple parties that depend on the data, and no one of them is trusted enough to have full control on the database. (trust-less scenarios)
Apart from Bitcoin[0] check out the original trustless timestamping service proposal [1].
[0] https://bitcoin.org/bitcoin.pdf [1] H. Massias, X.S. Avila, and J.-J. Quisquater, "Design of a secure timestamping service with minimal trust requirements," In 20th Symposium on Information Theory in the Benelux, May 1999.