Well, Oracle is more prevalent in big enterprise. But if you look at the equations at the end of the article, a mysql/postgres DBA costs probably 80-90% of the Oracle DBA. The licensing costs are less, but the hardware cost for the big-iron and big-SAN to run the RDBMS is pretty much the same.

I don't believe MySQL and PostgreSQL have much to offer for working with great big heaps of archival, unstructured or semi-structured data. Assuming that is the case, then it's appropriate for the article to ignore that they exist.