Ok, was able to find the cause. The Joomla package installer script has this check. The error wasn't visible in my scans since I was scanning Joomla and not GIT as Joomla does not copy over the Joomla package installer script (odd behavior compared to other installer scripts). Currently the only supported database drivers are mysql, mysqli, and pdomysql. It's possible we can enable installation on PostgreSQL, but can't guarantee it will work. A lot of Joomla extensions also do not support PostgreSQL so there's no telling what exactly will or won't work.

You can remove the check yourself if you want to give it a try. You'll need to extract the packager zip and in /extensions/packages folder you should see CBs Joomla package zip. In that you'll need to modify and replace pkg_install.php. In the checkRequirements function simply remove the checkDbo check.