Well, if I read what it says, IC operating carrier allowance applies.
In partucular, SK allowance to US in economy from last month is 1 piece/23kg, eco extra and business 2 pieces/32kgs each.

As a pure speculation/guess, you may have some luck to use EBG allowance on the trip ex US, when you will be checking in with SAS. On outbound trip with first 2 sectors by BT - I wouldn't count on it, but anyway please post what BT will reply to you.