I think he's just giving credit where credit is due. I read it as "I'm making a generalisation about in-memory databases, but VoltDB, which people would associate as an in-memory database, stands out as having other worthwhile innovations."
The topic of discussion is in-memory databases. The post made a general statement that in-memory databases are very similar to disk-backed databases. Voltdb is called out as an exception.
The guy who created Postgres also created VoltDB, so I'd imagine that most of the Postgres community is already familiar with it from watching his talks. (Which are worth watching, fwiw.)
