The reason why BA does not accept the designation on the boarding pass is that your record shows your status when you purchased the ticket, not your status when the boarding pass is printed. So, if you bought the ticket in February, when you were Platinum (not "Platnium") and then you fell to Gold on March 1, your boarding pass would still say Platinum.

As we all know, any computer document is easily created, so there is no reason for BA to accept that.

The moral of the story: when the entrance requirement specifically requires you to have your Platinum card with you, hang onto it. While others may not follow the rule as clearly stated on the Oneworld site, BA does.