The 50% chance he's referring to is the chance that, out of all that hashes that...

DennisP on Oct 25, 2017 | parent | context | favorite | on: How Merkle Trees Enable the Decentralized Web

The 50% chance he's referring to is the chance that, out of all that hashes that exist, there will be two hashes that match. He's saying that in his scenario it would be millions of years before there's even a single duplicate.