It's a much better situation than NULLing them out, because that hides bugs and makes tools like Address Sanitizer useless. A dangling pointer, when freed, will often throw an assert in your allocator; here's an example of how this looks like on my computer:

  $ clang -x c -
  #include <stdlib.h>
  
  int main(int argc, char **argv) {
      char *foo = malloc(10);
      free(foo);
      free(foo);
  }
  $ ./a.out
  a.out(14391,0x1024dfd40) malloc: *** error for object 0x11fe06a30: pointer being freed was not allocated
  a.out(14391,0x1024dfd40) malloc: *** set a breakpoint in malloc_error_break to debug
  Abort trap

As you turn up your (automatic) checking this will be caught more and more often. Setting the pointer to NULL will silently hide the error as free(NULL) is a no-op and nothing will catch it. Thus, the suggestion here was

1. advocating adding additional code, which has historically been hard to actually do in practice, and

2. providing a suggestion that is generally worse.