If we go on that, then you'd have to find another event that occurred at similar rates in similar geographic areas.
Now, if only there were medical indications that lead causes violent behavior... the article only points to IQ issues, though there is likely more evidence elsewhere.
There are lots of events that correlate with income (thus, with the geographical distribution). And lots of them correlate with the fall or raise of crime in the article.
Getting an event that correlate with both raise and fall of crime, like lead intoxication does, will be harder. But I doubt it's impossible.
The thing is, we have a causal nexus, we have the correlation, but we have no idea how strong the nexus actualy is, so we can't conclude the correlation is caused by the nexus. And we won't discover, because we won't conduct the kind of experiments needed for it.