At first, I'm not sure if the link  is realy about the SYSLINUX. In my opinion, Loadlin is rather different program, which can boot Linux kernel from DOS/Windows operating system, while SYSLINUX is "primary" boot loader. Now, ldlinux.sys is the executable which can run the Linux kernel. Important fact is that this file must be copy to disk (boot device) only once during instalation. It's HARDWARE adress is stored in the first sector of device. System will not boot, if you delete this file, even if you copy it back on disk later.