When SK is buying the seat on the BT flight they accept the service concept of that BT flight. And BT will not pay for the lounge access in this case if the pax is in M being EBG or not.
This again illustrates the dilemma with the current code share model within SAS (and *A) where you buy for instance a SK flight on SK ticket stock but end up with a different product than waht you might expected (better or worse).