Who do you believe? Apple? The FBI? Or the Hacker?
I tend to believe the Hacker for the following reasons:
If it was acquired by the FBI from Apple apple would, for business reasons, not disclose and further, the FBI would likely have required Apple be gagged as part of the order to turn over the information. Either way - Apple must deny whatever the truth.
The FBI is similarly in the position that they must deny because it would be a public relations debacle, it would be information germane to an investigation that they cannot comment upon, it would be a matter of national security, it would reveal methods, etc.
No information content in statements from either of those parties - leaving only one source that contains information. The hacker.
Are the UDIDs actual and real? Could they have been faked? Would they have been? Is there enough information content to say that the source is really a list of UDIDs with personal information? If so then they either came from Apple, or a trusted partner of apple or a massive scouring of net traffic or an FBI laptop. If they are real they came from somewhere. Wherever they came from, if the hacker announced the true source they would receive all the cred they care about. If they breached Apple, they would boast about how Apple security sucks. If they gathered the information from public networks they would crow about how awesome they were to do this. If they got the info from an FBI laptop they would piss themselves with glee that they found all this on an FBI laptop and tell the world. If they got it from a third party developer they would crow about how third party developers could get all the UDIDs they wanted. If it was a competitor of Apple that just wanted to embarrass Apple by stealing and then leaking the information then they would say they got it directly from Apple.
On balance I would believe the hacker.
There is another possibility: the data may or may not be genuine but the hacker actually acquired the laptop from the FBI but the guy they acquired the laptop from actually IS an FBI agent and the FBI just wants to see where the information leaks out from. If that is what is going on then why use real Apple UDIDs and would Apple have agreed to use their information for a sting?
Basically - if the information is real then I believe the hacker.