Is TCP not reliable?

I am believing TCP is reliable. If write(socket, buf, buf_len) and close(socket) returns without error, the receiver will receive the exact same data of buf with length buf_len.

But this article says TCP is not reliable.

A:

  sock = socket(AF_INET, SOCK_STREAM, 0);  
  connect(sock, &remote, sizeof(remote));
  write(sock, buffer, 1000000);             // returns 1000000
  close(sock);

B:

int sock = socket(AF_INET, SOCK_STREAM, 0);
bind(sock, &local, sizeof(local));
listen(sock, 128);
int client=accept(sock, &local, locallen);
write(client, "220 Welcome\r\n", 13);

int bytesRead=0, res;
for(;;) {
    res = read(client, buffer, 4096);
    if(res < 0)  {
        perror("read");
        exit(1);
    }
    if(!res)
        break;
    bytesRead += res;
}
printf("%d\n", bytesRead);

Quiz question - what will program B print on completion?

A) 1000000 
B) something less than 1000000 
C) it will exit reporting an error 
D) could be any of the above

The right answer, sadly, is ‘D’. But how could this happen? Program A reported that all data had been sent correctly!

If this article is true, I have to change my mind. But I am not sure if this article is true or not.

Is this article true?