That is not true. No matter what people say you can study for CASPer test and it is essential to do so.
Many students actually struggle when they decide to wing it. The scenarios are morally complicated and it is hard to come up with an answer on the spot. By practicing, you would develop your critical thinking skills and also practice writing fast that adheres to the qualities that CASPer is looking for.
When medical school looks at your application and if applicants have same MCAT, GPA, and similar extracurriculars, the CASPer score can also be a deciding factor. I wouldn't want to risk my future in scoring poorly in a test. Would you?