Computational Complexity Of Breaking Information Theoretic Security
Here's an simpler but analogous problem that may illustrate what's going on:

Given that $X=Y+Z$ and $Y=5$, compute $X$.

The problem isn't that the answer is difficult to compute, the problem is that there isn't enough information there to give you any idea (any information about) what the answer is. That's what's meant by information-theoretic security.
(Note: what I've given above isn't actually an example of SSS, and if you actually try to apply statistical information theory to it, you'll run into trouble because the integers are infinite. If you're really worried about the latter issue, assume we're doing arithmetic modulo some integer $&gt;5$)