It just says 'no optimization flags' and that's that, I don't think even the compiler is mentioned so the author is not giving you a lot to go on here - you don't even know what the default is.
A modern C optimizing compiler is going to go absolutely HAM on this sort of code snippet if you tell it to and do next to nothing if you don't - that's, roughly, the explanation for this little discrepancy.
PS: I do not use C beyond reading some of its code for inspiration, so kinda unaware