why the fd still available after shm_unlink() - linux

Question

I'm reading the source code in https://wayland-book.com/surfaces/shared-memory.html .
The author create a shared memory using shm_open(), and shm_unlink() it immediately, then ftruncate() the fd to a specific size, mmap() the fd and fill the region with pixels.
I'm so confused why the fd still available after shm_unlink().
according to the man page:
The operation of shm_unlink() is analogous to unlink(2): it removes a shared memory object name, and, once all processes have unmapped the object, de-allocates and destroys the contents of the associated memory region. After a successful shm_unlink(), attempts to shm_open() an object with the same name will fail (unless O_CREAT was specified, in which case a new, distinct object is created).
so shm_unlink() will cause the memory destroyed because there is no process mmap
the region. But how fd still avaliable?
here is the code:
static int
create_shm_file(void)
{
int retries = 100;
do {
char name[] = "/wl_shm-XXXXXX";
randname(name + sizeof(name) - 7);
--retries;
int fd = shm_open(name, O_RDWR | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
if (fd >= 0) {
shm_unlink(name); // unlink immediately
return fd;
}
} while (retries > 0 && errno == EEXIST);
return -1;
}
static int
allocate_shm_file(size_t size)
{
int fd = create_shm_file();
if (fd < 0)
return -1;
int ret;
do {
ret = ftruncate(fd, size); //why the fd still available?
} while (ret < 0 && errno == EINTR);
if (ret < 0) {
close(fd);
return -1;
}
return fd;
}
//after above, there was mmap
uint32_t *data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);