This is correct. The Multisource enabled universe is very limited in its use and it has not changed or been developed further since its original release in BI 4.0.
It performed absolutely horribly though. Even if you were just reporting against a single source, if you had the universe configured for multi-souce. We tried to use it with our Audit database reporting because we wanted to be able to join that data with data from the Metadata Management tool but the report just would not run. They run much better after switching to a single source universe.
I remember when it was first talked about and we predicted that the six months after it launched we’d be knee deep in queries on BOB of “why is my query slow on a unx” type questions.