Suppose i get my GC based on future employment offer from company A (I never worked for this company - my I140 got approved for this company). I used my AC21 and submitted another future employment offer from Company B to INS(I never worked for this company too- my I485 got approved shortly after using AC21).
So, i never worked for company A who sponsored my GC and under whom my I140 approved.
So, the conclusion is "After AC-21 introduced, there is no need to work for the sponsored company".
My attorney agrees for the above statement, but still he says...it is a grey area.