Often the answer to "some very clever language" is a lazy functional language like Haskell, where it's quite common to express problems naturally on infinite or near-infinite lists (of numbers, users, whatever...) in this way - and the language's lazy evaluation semantics effectively turning it into an efficient streaming data pipeline of sorts.
But even if not, the example from the article is just hypothetical. `db.getUsers()` could be something that just retrieves rather efficient `[UserEmail, ExpiryTime]` pairs, and then you'd have to have a pretty enormous user base for it to not scale (a couple of million string/date pairs in memory should be no problem).
But even if not, the example from the article is just hypothetical. `db.getUsers()` could be something that just retrieves rather efficient `[UserEmail, ExpiryTime]` pairs, and then you'd have to have a pretty enormous user base for it to not scale (a couple of million string/date pairs in memory should be no problem).