I've heard this argument before, and while there might be something to it, I think it's more specific to say that ambitious people tend to have fewer children. This makes sense purely in terms of time commitment: the more time is devoted to raising children, the less will be devoted to one's startup, research, political ambitions, etc.

This doesn't necessarily mean that more intelligent people will have fewer children, as it's more of a question of prioritization than of ability. It does probably imply a weak correlation (smarter people tend to like to use those smarts), but there are certainly many intelligent people out there who value their children more than they value their personal ambitions. (While this is certainly not a given... consider your parents!)