I should have qualified it as "RDBMS" server install. Most new SQL-centric web frameworks support three Open Source RDBMS engines: 1. MySQL, 2. PostgreSQL, and 3. SQLite. SQLite is rarely considered appropriate (arguably) for the server-side deployment RDBMS but its value comes from being embedded in the language runtime or web framework so it is available during development. IHP includes PostgreSQL as a Nix dependency so it is as ubiquitous as SQLite in the developer environment. "Out-of-Band" is relative to the project dependencies.
Using PostgreSQL in the development environment for say Rails or Django requires "out-of-band" installation, configuration, and management. Frameworks like Rails and IHP apply the schema/migrations to a live development RDBMS and read the Catalog Metadata to generate the classes/structs used as the Data Access Objects (e.g. ActiveRecord in Rails). The RDBMS, therefore, is part of the CI/CD pipeline.
Other ORMs define their entities independent of the RDBMS. I think Java JPA and Apple Core Data work this way; the SQL DDL is generated from the Entity Model. YeSQL-style frameworks also require a live development RDBMS but they extract metadata from each prepared statement rather than catalog metadata for each table which is mapped to a class/struct.
I think sradman means that Nix enables IHP to guarantee the presence of postgres, rather than relying on the user to ensure pre-requisites get installed.
