Assange makes the point that while it is intellectually entertaining, the average person will still have to trust expert analysis to see the proof as valid.
Since the blockchain is a public cryptographic ledger, any miner can add data to it, assuming they include a "proof of work." Bitcoin uses a bit of math call SHA256 to generate a fingerprint of the transaction. It's "one-way" in that it is difficult to create a hash, but very easy to verify.
Personally, I think we can take this as proof he's alive. I think it's interesting that people need this level of verification. I guess that's just how nervous people are these days. It also demonstrates the sorry state news organisations are in. The only person we trust right now needs to show cryptographic proof he's alive. The AMA has ended and you can skip to 1:53:00: