There's much debate about whether or not WordPress even qualifies as a CMS. I'm not going to entertain such debate, but WordPress was designed and built with blogging in mind; it's highly optimised for such a task.
Joomla! has blogging capacity secondary to content management and in my opinion, is much better quality too. WordPress is the easiest system to use and therefore most widely used. People are just lazy.
There's not really any good reason to use WordPress over Joomla! even if one only requires a blog. The blogging abilities of Joomla! are comprehensive and complete. People often make the argument that Joomla! is unnecessary (too big, extensive, etc.) when WP is optimised for blogs and while this is true to some extent, the Joomla! system is no more complex on the back-end and is probably more intuitive in the end.
Regarding your question: I was implying that you wouldn't want to attempt to recreate the system and therefore, would be better off simply learning it. Also that you're not going to learn much about commercial PHP development from Joomla! since it's a closed API unusable on any other projects.