I think the issue here is a confusion about what "bias" means. If they are self-assessing at random, then the high performers will all underestimate themselves,

I think the issue here is a confusion about what "bias" means. If they are self-assessing at random, then the high performers will all underestimate themselves, but this is not a bias towards underestimation as they are choosing randomly. That said, the chart from D-K seems to show a different bias and line up roughly with what you would expect. Someone with no knowledge assumes they are average skill and hence inflates their position, someone who is very good doesn't want to rate themselves the best because they assume others know as much as they do. The assumption underlying both groups is that you are normal and others are similar to you. I hypothesise that most people think they're average, which is something you could easily test by asking them to rate how well they think the average person would do on a test and comparing it to that individual's test score. I'm almost certain that high performers will overestimate the average, and low performers underestimate it.