This is true relative to their own ability. A highly competent person may consider themselves to be more skilled relative to average than an incompetent does, but the highly competent person will most likely still be under-estimating his ability and the incompetent over-estimating their ability.

This is critical, because it means thinking you suck doesn't imply that really you are good. A 95% percentile person might think they are in the 60th percentile, and so might a 5% person. Both have the same opinion of themselves.

Another critical point is Dunning-Kruger hypothesized that for highly skilled individuals this effect was due to their lack of awareness about how relatively unskilled others were. When they were given a chance to look at other work they were able to more accurately assess their knowledge.

One possible control is instead of asking individuals how good they are would be to show individuals existing code and ask them how good _it_ is. Dunning-Kruger would suggest that incompetent people would be too ignorant to accurately assess it, and most likely over-estimate its skill, whereas skilled people _are_ able to accurately assess fault in others' work.