I think this is a new offer.
There was a time when the bmi website said that the MBNA credit cards would earn double miles when booking bmi flights when they were moved to ba.com (flights BA8xxx and BD codes), but I tried one of my bmi bookings on ba.com with my MBNA card and never got the bonus.
Now it sounds like the offer is double miles on anything bought on ba.com (presumably car hire and hotels too based on the wording). This will be easier for MBNA to implement.
It'll be great for those of us with the bmi Plus card as we'll get 4 miles per pound on BA spend (better than the 3 miles/£ on BA spend with my BAPP card).
It'll also be good for those with the regular bmi cards who didn't want to pay for the BAPP card.
I can easily spend the £10k required for the BA 2 for 1 in two months, once I'd qualified for my 2 for 1 I used the bmi cards for everything except BA flights, now even for BA spend it'll be the card that I use!