On 4/3/16, Griffin Boyce
How do you transmit an elephant? One byte at a time...
But on a serious note, it's possible to transfer 2.6TB over Tor in small pieces (such as file by file or via torrent). Given the size, however, I'd suspect they mailed hard drives after establishing contact with journalists. Even on a fairly fast connection, 2.6TB would take quite a while...
That amount of data would take 27 days at 10Mbps. Few would be willing to sit supervising in a hotseat that long when they can physically mail 3TB for $100 and 8TB for $230. Though they might spend 3 days pushing 100Mbps via shells, etc. Overlay networks move data reasonably well, and reliability could be handled by chunking protocols. Available link speeds (thus path speeds) are likely to be limiting factor, ie: 10Mbps limits you to 100GiB a day. Though at 1Mbps, DVD torrenting on say I2P seems to be a thing.