Great question.
It's similar to the questions:
1. If the faithful and discreet slave is really Jehovah's channel of communication in these last days, exactly how does he communicate with THEM?
2. If the faithful and discreet slave is really Jehovah's channel of communication (in a way that no other religious organization is), why have they gotten so many things wrong that needed to be changed?
3. If the revelations the faithful and discreet slave receive from Jehovah are progressive truths (getting brighter and brighter), wouldn't they start out as true but incomplete rather than as totally false?