It's all in the timestamps. They line up, almost to the second, that showed a transfer speed @USB2 max speed - yes. Bill Binnie, with the help of another company tried to transfer across the Atlantic, to various destinations, and didn't get even half that speed. It's all in the report, educate yourself and read it, even the FBI said the conclusions they reached couldn't be completely verified.
Ok, so "almost to the second for usb2 max speed" (IIRC about 60MB/s with no allowance for overheads) except you don't get that unless you're transferring a suitably large file to a device that has a fast enough write speed and there is nothing else going on with the system (the source drive, the destination device, the USB host).
Try transferring some files from a computer to a USB device (hard drive, memory stick etc), tell me if it stays at that theoretical USB 2 max speed for the whole process - it's pretty unlikely it will even for large files
Then some nonsense about them not managing to get half that speed across the atlantic is a really poor connection, and worthless as a test unless they're actually using the same server/hardware and it's done at the same time.
Of course the FBI are going to say their conclusions can't be fully varified, any decent investigator will put in qualifications if they can't be 100% certain of every aspect, that doesn't mean they're completely wrong in every aspect and in the most basic parts of their conclusion - they may have proof that the server was accessed illegally by an IP address, and that the information was transferred to that IP, they may have evidence that it was most likely by group A but if they can't prove that they'll put in the qualifier to cover the fact they aren't certain it was who they said it was, that doesn't mean they aren't certain about how it was done.
On the flip side it's so much more proof that it was done via a usb stick because someone looked at the data transfer speeds (I'm not sure most servers even log data transfered by USB connection in the server logs...) and because they couldn't duplicate that speed using a completely different server under different conditions at a different time.
That's like saying that the a car's claimed 0-60 time on a race track in the summer isn't possible because you couldn't do it in a muddy field whilst towing a caravan.