XL1 covers are not flat they are "convex" as you can see from my video shot at the vw dealer little while ago. They come out actually quite much.
There is other reasons than aero to manufacturers not use convex wheels. They get curb rash even more if spokes come outwards specially with those thin side profile tires. I have seen a study that concave vs flat wheels was about 1% penalty drag wise. Cannot find that study on my computer anymore though.