I couldn’t figure that question out, but I didn’t bother responding. I don’t know why someone getting an A in a prerequisite class thinks summer preparation is needed.
That’s the first time I’ve seen that question, so I certainly don’t see it as a “common assumption”.
Prep for AP is just the equivalent of studying for finals. Everyone in college does this, so I don’t see why doing it for what is supposed to be a college level course seems odd. Other HS courses rarely have tests that cover more than the last quarter, usually just the last chapter. So a year-end test that covers the entire year of material is new and I think studying for it makes perfect sense.
Subject tests cover a broad set of material, all of which is unlikely to be covered in any single class - they even state this in the test descriptions. I think it makes sense to study topics that you didn’t cover to maximize a score. My D learned vectors, matrices, and a couple other topics she didn’t cover in PreCalc for the Math 2 test. Her Chem teacher went over the list of topics covered on the subject test and pointed out those that wouldn’t be covered in class.
Yes, CB says scoring accounts for the fact that most students will have questions they haven’t seen, so it’s Ok to miss some. But if you can study those topics on your own, you increase the chance for a higher score. Given the opportunity, I don’t know why you wouldn’t.