You should read the posts you quote more carefully. I thought I was clear: it doesn't have to be the actual Kaplan diagnostic. It can be a previous MCAT, if you have one. The point is supposed to be to mark your improvement, not to try and get a course for free.
I mean, they don't have to offer a money-back guarantee at all. After all, you're paying for a class. Do you get your money back if you pay for a semester of college, never go to class and flunk everything? Sheesh. If you don't want to pay for a course, don't take one. I didn't, and I did quite well on the MCAT. There are no magic bullets. If you study the material and practice the form of the exam, you should do fine.
Truthfully, I think taking courses for standardized exams is kind of silly. But to take a test and "cheat" a higher score by cribbing the answers beforehand in order to beat Kaplan out of the cost of the course is pretty pathetic. Not to mention unethical.