That sounds unlikely to me, or at least something hard to measure that could easily be spun in different ways depending on your favored interpretation. Do you remember where you read it?

A couple of immediate objections come to mind. First, "intelligence" is difficult to define or measure. Things like standardized IQ tests tend to come with an implicit agenda. Second, thinking instead about "innovation", history surely shows that most innovation happens when humans group together (in farming communities, cities etc).

It's true though that in recent history, cities are more associated with social atomization. Overall it seems like a very fuzzy and complicated issue.