Lots to see when we find correlation, even if we aren't certain of the causation.
First and foremost, we should attempt to replicate it and see if the correlation holds up independently.
Second, we should attempt to determine if their is a causal relationship. The arrow of causality going in either direction is of great interest to urban planning.
Or third, if there is no causal relationship between them, we should attempt to determine whether there is a third correlating factor, or better still, another factor that has a causal relationship with these two.
This discovery is highly interesting, and points in the direction of useful further research to perform. I would definitely not say that there is nothing to see here.
Authors often do some research, and then present some conjectures. Their conjectures may not be supported by the research, but they point in the direction of further research to confirm or refute the conjectures.
Research like this may not be sufficient, but it may be useful.
There's another possibility: The two variables are completely unrelated but we only get to observe them when some function of both of the variables is true. The classic example is two independent coin tosses that trigger a bell when both coins come up the same -- and we only think to look at the coins when the bell sounds.
