seems like it obviously is though. i mean, the original german name that sounds phonetically similar to ljubljana might not be related but the slavic name obviously means just what it seems to mean.
in bulgarian we have expression. "looking for a calf under the ox". it means looking for a complicated explanation when a simple one will do. what do you think the odds are of a generic german name randomly morphing into an obviously slavic one with a clear meaning over the ages. not good right?
sure, we don't have concrete proof but what does your gut tell you?
anyway, as far as i'm concerned, the fact that even though you don't call it ljubljana, but lubena, and you still have an equivalent slavic term (lublena) makes it even more clear cut