Datomic "on premise" version is not dependent on any AWS services and can be run on any box using vanilla java

There appears to be an on premesis version, which doesn't use AWS services.

https://www.datomic.com/on-prem.html

That threw me away, worse is that the cloud version and on-premise are different products and not compatible. Is not something like RDS where you can just move your postgres setup to your own server or some other cloud service.

As it just happened to you, you can see they are putting some effort in hiding on-premise from their front page. All their effor seems to be going to the cloud version.

You can use a variety of different backends as the data store.