The way I understand it, these authenticators are factory programmed with a unique cryptographic key. When Blizzard buys a batch of authenticators from the manufacturer, they are also supplied with a list of keys+serial numbers that they add to their authentication server.

As Blizzard didn't buy a batch of FFXI authenticators from the manufacturer, they didn't receive the associated keys to add to their server.

You could probably try to supply the serial number of an FFXI key when you try to associate it with your WoW account, but Blizz's systems will probably just say "we didn't supply any authenticator with that serial number and we don't know it's unique key"