I, too, scratched my head over that for a while, and eventually realized that I had probably misunderstood what the authors are trying to say.

I think what they mean is that if the compiler has analyzed the code, not shown in the example, that comes before the if statement, and concluded that unless something undefined happens, cond can only be 0 or 1, then it can optimize out the condition.

They are not saying that the code actually shown is enough to conclude that cond must be 0 or 1.