He's saying they're all 'pre-existing' prophecies, and therefore JWs haven't originated any NEW prophecies and therefore aren't prophets. This isn't true however.
Example 1: The Great Tribulation & "this generation" - In this prophecy by Jesus, he clearly said that it had never happened before and 'nor will occur again', so they are making a new fulfillment.
Example 2: 1925 - This was based on counting jubilee years etc. and was not even based on a prophecy to begin with. Therefore, it is a wholely original prophecy, and was said to be 'confidently expected' to come true.
More examples could be cited, but all that is needed is one. Deuteronomy clearly and simply states that anyone who claims to speak 'in the name of Jehovah' and who's words don't come true is a false prophet and is to be ignored.