The shutdown addresses more qualitative aspects of the situation. Even if flattening the curve doesn't reduce the area under the curve, it still avoids a potential large numbers of otherwise avoidable deaths because fewer people have it at once, keeping hospitals under capacity, and more people will get it later when we know better how to treat it.

And the earliest predictions have to be the least accurate, because they're based on the least amount of data. But early on is when you need to act.