Well I certainly agree that having a "Show Desktop" icon on the desktop would be pointless. Near the beginning of the tutorial, did you see the Tip, How to Add Show Desktop Back to Quick Launch?
Before proceeding, has the PC been restarted since the original icon went missing from Quick Launch? One of Vista's quirks is that missing icons sometimes reappear when the PC is restarted. But if the original icon was deleted and the recycle bin was emptied afterwards, then following the Tip is the only suggestion I have.