Module update repos need to be enabled, but that requires some work as per Owen's comment in https://pagure.io/modularity/fedmod/pull-request/74 :
This looks like it will require changes to how _write_lookup_caches() reads module info - though I'm not entirely sure how the modules from the modular updates repo should be merged with the modules from the base modular repo - I'd guess if there's a module with the same name:stream in the updates repo, it should replace the module in the base modular repo entirely.