NULL doesn't exist in assembler, and in C, NULL is only a defined as a macro. It's not something built-in.

C doesn't have any problems adding 4 to NULL nor subtracting NULL from NULL.

"Having problems" is not a fair description of what's at stake here. The C standard simply says that it doesn't guarantee that such operations give the results that you expect.

Also please note that the article and this whole thread is about the address zero, not about the number zero. If NULL is #defined as 0 in your implementation and you use it in an expression only involving integers, of course no UB is triggered.

  #include <stddef.h>
  int main() {
    if (NULL + 4) {}
    if (NULL - NULL) {}
    return 0;
  }