Are dimpled chainstays weaker than smooth ones, other things equal?
The dimpled chainstay may have less stiffness in certain directions at that point, but if forces are not concentrated there, or in the direction of less stiffness it just doesn't matter. 

A u-shaped beam (which is what the heavily dimpled tube essentially is) has less resistance to twisting forces than a tube, but there isn't any significant twisting forces on the stays.