[rumdog]

So connecting the dots here...

For the ultimate drag reduction, build a VW XL-1.*
...with dimples!

So it looks to me like the dimples (or grooves) create a subtle turbulence within the layers close to the body. As opposed to VGs (vortex generators) which create an aggressive local and downstream mixing action using a vortex.
Does the non smooth surface (NSS!) also reduce the thickening of the boundary layers?
It is interesting how the turbulence (rear bluff rotating eddie) is reduced by the NSS [figure 8] and it almost looks like a vortex develops on each side... disapating energy as vortex instead of rear eddie. Or what?

I think that other body shapes may not be so lucky. Or just need engineering too.