At the beginning of the article, the "relational" database is described as a particular architecture ("map of maps, with secondary indexes being additional maps

At the beginning of the article, the "relational" database is described as a particular architecture ("map of maps, with secondary indexes being additional maps") without recognizing that the other given examples ("key/value", "document" and "column-oriented") are also relational and the differences are only, at worst, performance tradeoffs. The more I read it the less it makes sense.