I think the idea is that people on the lower mode of grading would self select themselves out of a CS at the 100 most often. This reduces the bimodality for the 200 level. Where again students self select themselves out, then the 300 level bi-modality is even further reduced.
I think the 100 level is the place where such a bimodality is most likely to exist and investigation ought to be focused there.
Your parent's point is that this hypothesis ("bimodality reduces as students progress through the curriculum" is possibly contradicted by the fact that many late-stage mathematics courses are strongly bimodal.
Based on some limited experience upper-level math and (I assume) physics probably have a stronger element of "get it" vs. "don't get it" than most majors. Way back when I was an undergrad a number of the math majors I knew considered it to be a relatively easy major and there were definitely people who pretty much breezed right through.
At the same time, I was quite convinced that I could no more have successfully graduated as a math major--no matter how hard I worked--than I could have successfully flown through the air by flapping my arms. By contrast, I'm reasonably confident that I could have gotten through pretty much any other majors, whether engineering or something else.
I think the 100 level is the place where such a bimodality is most likely to exist and investigation ought to be focused there.