What that boils down to is this: WordPress is blogging software that has features and a plug-in infrastructure that allow you to expand it until it becomes a sort of CMS if you need one; Joomla and Drupal are content management systems that will allow you to construct (or install) a blog if you want to.
Once upon a time, WP was just a mediocre blogging platform¹ that could be hacked by a developer with enough patience and ingenuity into doing a little more, and both Joomla and Drupal were essentially programming languages on top of PHP with a built-in publishing workflow engine that didn't do anything until you built an application on top of them. These days there is little you can do with the "real" CMSs that you can't do with WordPress and a few plugins, provided that the aim is website publishing and you don't have a complicated set of permissions and approvals to deal with. If you have a large writing staff and more than one level of editorial approval to go through, or several different private/collaborative areas, then WordPress is going to be more of a problem than a help. (Neither Joomla nor Drupal make it easy, but they do make it possible.)
¹ Early WordPress really wasn't that good... but it was free, and TypePad, its main rival, was not.