It would be hard to overstate how incorrect this statement is, if it is read with the implicit qualifier "for all possible inputs", without which my comment above would be obvious nonsense. Of course we can tell what most programs will do for some inputs—we can just run them!
Yup, we can tell what most existing software will do for all inputs. Rice's theorem states that we can't tell what all software will do, not that it's impossible to tell what a given piece of software will do.
The fact that we can determine what a piece of software will do, doesn't mean we always do that kind of analysis, or that the programmer fully understands his own code. That's why we have type systems, constraints, verification tools, etc.
