The article has nothing to do with optimizing compilers, nor requires knowledge of them. It merely uses optimizing compilers to illustrate what is and what isn't legal C. When there's undefined behaviour, the compiler has almost free reign, so it's important for a C developer to know what is and what isn't undefined behaviour.

My boss who is a C developer had only two answers wrong, he didn't remember you could tentatively declare a global var, and he was fooled by the comma operator inside the array index.