With a little experimentation, the member_id field is specific to you, that is why every one else is getting the "already on record" message.

Similiarly, with a little modification, I managed to "find" my member_id and tried the link without success. This "offer" is only good if you haven't updated your Cybertraveler Canada profile with your AAdvantage number yet, which also explains why I didn't receive this offer as well.