The $merge requires to throw NamespaceNotFound when a db doesn't exist.
In case the router is stale, the NamespaceNotFound wont' be thrown and the db will be created.

Extra context
Implicit creation of the database is disallowed due to an old constraint of the server, where cluster writes would not create the db. I inferred this by this old conversation from the original commit of the test. This restriction seems to no longer exists, however, the NamespaceNotFound is a consequence of this forced refresh, required to establish some checks on the indexes.

Whether throw NamespaceNotFound or not should therefore be deferred to SERVER-95749