With Wikiscan I can see stats of users whose userpage is listed in a specific category.
How can I see all contributions by Wikipedia users in a specific category? The category only contains their user pages.
You want the Special:Contributions list for users listed in that category. And indeed this is not something MediaWiki core offers. There is however a tool by Magnus Manske, Herding sheep, which produces list of edits by a list of users.
The toolbox item "Related changes" lists all recent changes to pages in the category (it's provided by MediaWiki core).