The only reference I can find to the signature cards is on Jeff Allender's checklist site, where he checklists a red bordered Arguello. Since it references the border color difference I think the report of the Arguello is credible. Allender claims (without foundation) that the insertion ratio is 1:2160 packs, but is "reportedly scarcer." Yeah, no $hit, Sherlock. A total of two cards identified in the hobby over 27 years is definitely scarcer than one every 60 boxes (5 cases). I'm thinking that finding one is more akin to this scenario:
As I recall from reading Shark Tank, when Upper Deck invented the signed insert around this time, the executive who came up with it was inspired by Willie Wonka and wanted it to be like pulling a golden ticket. If Kayo followed suit, the cards may be unique. We didn't really start thinking about larger hit counts until years later.