What a flawed, sensationalist headline. So, he looked at all people that used aliases for Gmail accounts (added a +something or . to their address) to register multiple times.

By comparing the hashes for these Gmail users, he then determined that 51% used the same password for duplicate Adobe accounts. He asserts they were "highly technically savvy users" and thus 51% is the lower bound for the population of all Adobe users, thus 51% of the users used the same password somewhere else. Therefore "64 million" user's accounts are compromised on other sites, because they must be using the same password else where.

OK. Here is the problem. You, a savvy user, have multiple accounts, on the same site, why would you use different passwords for your alias accounts? Same password on same site != Same password on all sites.