"Batteries Not Included" is a phrase found on the box of children's toys, so that parents know they will need to buy batteries before giving it to their kids. It has become an expression for something being incomplete and will require effort before it will work properly.

In this case the author is stating that Postgres doesn't come with a high availability capability. He then goes on to explain the high availability setup he put together.

"Batteries Not Included" in this article refers to the fact that PostgreSQL doesn't offer any built-in solution yet to do HA (automatic fail-over) and you have to depend of 3rd party solution.