Yes, but they (cul-de-sacs) are clearly designed to prevent cars from going fast, which is what makes them a good counter-example for "everything about American cities makes more sense when you realize they are designed entirely for cars to go fast."
People want to walk their dogs and let their kids play in the yard and they're pretty aggressive about trying to prevent fast through traffic where those specific things happen.
Sure, but they're a reaction to "designed for cars to go fast". As a general statement, we do in fact engineer our roadways with maximum travel speed as one of, if not the, top factors. And then we design closed-off culdesac neighborhoods to limit our exposure to those fast roads. Where instead we could simply prioritize other transit options and other neighborhood designs (put shopping and schools within walking distance, with appropriate infrastructure).
People want to walk their dogs and let their kids play in the yard and they're pretty aggressive about trying to prevent fast through traffic where those specific things happen.