Confirmed. Any flight that isn't on BA, such as flights I could have taken on Aer Lingus to complete my trip from IAD to DUB on my current trip would not have allowed the BA 10% discount to be applied. You can check whether BA flights are involved by looking at the flight details while going through the booking process--there are definitely codeshares with AA and IE that will not attract the discount.

Same principle applies to the AARP discount--only on BA metal.