I'd argue that implies you don't know where string theory fits in the "pantheon" of physics. It applies to the realm well below the radius of the proton, required to explain only the most exotic times and places (like the moments after the big bang, or the boundaries/interiors of black holes). Carroll's point is that basically ALL of "everyday physics" is known - everything above the radius of a proton, which governs all the stuff and signals we are and deal with in our solar system and local chunk of galaxy. String theory is an example of a "weak" theory because it's not unique, but it also applies only to exotic things and it's haziness does not affect our understanding of the larger regimes.