Joomla and Drupal are both Content Management Systems (CMSs). I can't speak for Joomla, but I do know that Drupal is a consumer of CMIS. That means it can easily create, query for, update, and delete content that resides in a CMIS-compliant server. Using CMIS is a very common approach for integrating Drupal and CMIS servers like Alfresco.
These systems could also be CMIS-compliant servers, but someone would have to do that work. They are not CMIS providers out-of-the-box.
subject: Can Joomla and Drupal be considered CMIS too?