He claims that the FBI already has all of the suspect's communication records as retrieved via service providers, but ignores the important detail that iMessage uses end-to-end encryption, rendering any such records unobtainable by the service provider.
Yes, the last backup was six weeks prior to the phone being seized, but this only means that the phone may well include six weeks' worth of pertinent evidence. And there is nothing to suggest that the FBI is only interested in messages between co-workers.
Ultimately, the phone's data would need to be decrypted and analysed to see if the first four assumptions that Snowden makes are actually true.
The final assumption is that there are other feasible technical measures that could be taken to crack this phone. This would perhaps be the most interesting point, but Snowden chose to not expand upon it at all.