The approach of the WTS was always that you should put the claims of other religions to the test, but when it comes to their teachings, you are expected to verify that they are true.
For other religions: test the claims and see if they are true.
For the JW teachings: read the claims and you will see that they are true.
The GB considers itself to be the voice of God, essentially. To test/question their claims is to doubt God Himself, as far as they are concerned.