The first migration from ms sql to mysql was licensing costs, and the desire to host on Linux. The more recent plan to move to postgres is motivated in part by features and in part by standardizing on postgres.
Well, that is another area the article got partially wrong. There are many postgres specific features included in Django, and other available via third party packages. And for most things that the ORM doesn't specifically support, it gets out of the way and lests you do them anyway, by executing your own custom SQL, right from the ORM.
