MediaWiki does not make this assumption, and DB connection code can handle the full string dbname / db domain ID and work as expected.
Wikbase however currently makes some different assumptions throughout the code base meaning false MUST be passed around in order to refer to the local DB.
This makes the work with EntitySources hard in terms of configuration.
Ideally we would deifne a set of entity sources for a cluster such as Wikimedia, and those same sources would be used on all wikis.
However due to the current assumptions we need to define different entity sources on the repo vs clients, as repo needs a false db name and clients need the string.