This is definitely outside the scope of my knowledge, but if you dereference a null pointer, won't you get an illegal memory access?

Undefined behavior, at least colloquially, is about dereferencing a pointer to memory that may or may not be accessed and may or may not work/crash-the-program. That's what makes it undefined and an absolute disaster to track down.

I'm pretty sure that if you try to deference memory address zero the OS will bark at you.

Again, it's not my area of expertise, so please correct me if I'm wrong