@kmk, the plugin works as intented. The problem in your case is that you are using Group Collaboration and wants to list tags from its topics.
My plugin was made and it's compatible with Core/Forums app only.
By default, it only searchs for tags in forums. You can include or not Clubs, so GC topics will be iincluded anyway.
It uses the basic permission getItemsWithPermission, so I'm not sure if GC extends that in anyway to change how it works. It also only shows topics that everyone can see (it exclude topics that only author can see). So if it is like that, I suggest you to show this simple plugin to the GC dev and see what he says.
I may add this a new setting in my plugin.