They are explaining things wrong. That option is a modifier of the
Limit the output of depends and rdepends to packages which are currently installed. Configuration
But then, it just don't show that, but is the list of the package that are installed, i. e.:
$ apt-cache depends kde-baseapps-bin
As you can see, all the dependencies of the package, if I use
apt-cache --installed depends kde-baseapps-bin
Shorter, right? Out of all dependencies of that package, those are the only ones that I have installed. That's what the
--installed modifier do, it do not list all the installed package.