Actually, no. If at least one user from your instance has subscribed to a community of another instance, that other instance’s communities show up in your search results (they are federated)… at least that is how it should work 100% of the time, but it doesn’t. In most cases, it doesn’t actually show all of the communities that would show up during a search, meaning the web UI would return a lot more results.
The method you’re using is actually a safe bet: specify the instace’s URL and don’t have to rely on whether that specific instance is federated with my instance, just search for communities there as well. That approach is easier if you actually have a link to the community which you’d like to add and the instance on which it resides. But, if you have no idea on what instance that community might reside, it’s easier to actually seach for it via the web UI, which I hoped would be implemented in an app sooner or later (having relevant results I mean).