I think 'ars is saying that these are "personalized" email addresses: there is a separate one for each person from whom 'ars wants to receive email. Assuming these addresses aren't easily guessable/enumerable, and aren't on any lists stolen from or sold by service providers, the spammer must have have gotten them somewhere else.
The problem is that pretty much every one of them have probably given one or more services access to download their contact data to connect them to their friends, so any number of services other than their e-mail likely contains these personalized e-mail addresses.
Someone has likely been hacked or sold/leaked data, but he should assume that those addresses have been spread quite a bit voluntarily by his friends/associates.
