I think WordPress just tries to cut complexity by using MySQL only so it doesn't...

TazeTSchnitzel · on May 23, 2013

More likely, the code is poorly written and heavily reliant on MySQL.

jacques_chester · on May 24, 2013

This is more or less the correct answer for Wordpress-the-ecosystem.

Wordpress-the-base-install has a thin abstraction layer over the database in the $wpdb object.

But plugins and themes can, and regularly do, use PHP's MySQL functions to access and manipulate the DB directly.