While this appears to be made in both jest and political commentary, I agree that there are bounds to human intelligence.
While we as a species have accomplished a lot, those accomplishments seem to be sparked/possible by the few of us who stop and figure things out or notice something interesting. It seems that most of us take most of the knowledge we have for granted; Even the most essential pieces of our knowledge are passed down, not figured out, by the vast majority of us.
Examples: fire, sanitary practices, what to eat (and what not to eat), language (and writing)
How I justify these examples:
It took a long time (many generations) to harness fire and still more to understand it well enough to use it as we do today. We are still pushing the limits of our understanding of fire (combustion) in microgravity to better understand how it works on Earth.
Sanitary practices are different in different cultures despite all of us having the same basic plumbing. Sure, we can all "poo in a hole" but even in modern times, we've had to re-learn to not make that hole too close to our water supply.
Food: There is no way of knowing what mushrooms will kill you without someone previously taking a hit for the tribe.
Language seems pretty self-explanatory but... there are languages we no longer know and are largely unable to decipher because we have stopped passing them down. Knowledge encoded in those languages may be lost until rediscovered in a new language.
I think the biggest bound to our perceived intelligence is the ability to pass it down. When viewed in the scope of animal intelligence, we often think animals that have learned from other animals are acting more intelligently than those that haven't. The problem is, both groups may be equally intelligent, one group is just more highly trained.