If we're talking real-world languages, the code is perfectly well-defined. It's undefined behaviour in Abstract C, the C of the Specification, which to my knowledge nobody has ever implemented, but Quake 3 was not written in Abstract C. It was written in Visual C++ 2003, gcc 2.95, and some other specific C implementation.
The code snippet that the article claims is C:
int32_t compute_magic(void) {
double sigma = 0.0450465;
double expression = 1.5 * pow(2.0, 23.0) * (127.0 - sigma);
int32_t i = expression;
return i;
}