It's a chat log from the months after oct/2016 when JA went dark. I can attest to it's authenticity.
The programs in that chat log are simple python programs that poll blockchain.info for transaction data found within a transaction id. The original Jean.py script pulled data from a single transaction ID, but further iterations (jean3.py) followed a tree of transactions to generate much larger files. There are gigabytes of data immutably included in the Bitcoin Blockchain.
Hilariously enough, alt-coins that used a hard fork of the bitcoin blockchain, to seed their own, also contain the wikileaks data. (bitcoin cash, bitcoin gold, ect.).
In the example provided, they used the transaction number that contains data for the cablegate backup. Wikileaks shoved their entire proof of publication, AND DATA into the blockchain well before anyone knew to look. Well after it could be changed or manipulated. It's immutable. Never to go away. Forever existing as long as bitcoin remains secure.
You'll notice references to satoshi nakamoto's download tool which can be found within the transaction provided. This is a very simple tool that allows you to perform similar actions as the jean.py script.
This is a very deep and fruitful rabbit hole. Very reliant on technical programming knowledge and knowledge of the bitcoin protocol. Read between the lines. Use the examples as a guide for further research.