why pthread_join didn't block its calling thread

I am learning how to use <pthread.h>, and the textbook says that:

The pthread_join function blocks the calling thread ...

But in my test, it didn't:

void *a_thread(void *pt) {
  puts("a_thread:join");
  pthread_join(*(pthread_t *)pt, NULL); // <-- a_thread didn't wait for
  puts("a_thread:joined");              //     itself to finish executing.

  return NULL;
}

int main(void) {
  pthread_t t;

  puts("create");
  pthread_create(&t, NULL, a_thread, &t);
  puts("created");

  puts("main:join");
  pthread_join(t, NULL);
  puts("main:joined");
}

gcc -Og -o test test.c; ./test:

create
created
main:join
a_thread:join
a_thread:joined
main:joined

From the pthread_join man page:

If multiple threads simultaneously try to join with the same thread, the results are undefined.

Here you have the worker thread and the main thread both trying to join on the worker thread, sounds like UB to me.