The FBI is full of schitt.
They won't know until they try.
If some fool did a quick, high level wipe "format" and the area of the disk was not over-written, they may be able to find something.
If someone did a multi-pass low level wipe and then the disk was over-written it would be hard to recover anything.
They don't call it DoD Method for nothin'.

I think that the hard drives were physically destroyed and the servers got new hard drives before the seizure.
You have to grab that stuff quick and in this case there was plenty of time to do anything imaginable.