Preparation helps for any test, simply because tests have a specific structure, format, expectations for how answers are to be given etc. That doesn't mean that

Preparation helps for any test, simply because tests have a specific structure, format, expectations for how answers are to be given etc. That doesn't mean that test scores are indicators only of preparation. Even a native speaker of English might not get a full score on the TOEFL or the IELTS if they are unaware of the format etc. That doesn't mean the TOEFL isn't fundamentally a strong test of the knowledge of English. Another way of looking at it: Athletes need training and coaching, but that doesn't mean any random guy paired with a coach will become a top athlete.