It's all about trade-offs
The John Paul George and Ringo distribution map is excellent. This of course is precisely the same phenomenon as that of ability grouping in class or setting. It all depends on where we draw the lines, or the boundaries around groupings. Do you also consider the ability grouping model to be in need of drastic change?
A well-designed test should not rely on background knowledge. The knowledge should be either provided in the context of the passage or separately. This is a well-known test framework parameter. It will interesting to get a copy of the test to see if the complaints are valid or taken out of context.