I would like to understand first what could have happened to existing rocker cover? Can they warp? Doesn’t look to me like it’s worn at the filler seat.
If it was mine I'd be inclined to ignore the dealer, take out the little assembly myself to see what's going on. It's dead easy. I'd hazard a guess that the seat is damaged... rather than being oval/warped/ buggered rocker cover. For example, you might have had old O-rings that we'r causing the leak and just needed replacing, but a heavy handed mechanic damaged the seat when removing the old one?