The wiki entry evidently doesn't describe what it does because according to the wiki entry it allows for the enabling and disabling of preferences. The updating of a certificate is beyond what is described in the wiki.
Mozilla should follow up with a post describing exactly how Normandy works and the full capabilities it gives them.
From what I understand, Normandy is an infrastructure for delivery of some changes to some of Firefox users (or all of them). There are two major use cases: preferences rollout and studies. In the first case default values of preferences get changed (if your pref has non-default value, it won't affect you). In case of studies some piece of code gets delivered and executed, which cat do anything. In this hotfix the study installs add-on, which in turn installs certificate.
Users shouldn't have to search and then be able to understand the code found for such a feature. When a remote capability such as this exists it is Mozilla's responsibility to document how the feature works and the exact capabilities it gives them. Instead of doing so they have produced a wiki entry which appears to falsely describe the capabilities of this remote feature by stating it is used to change default preference values.
Also here's the code for the server: https://github.com/mozilla/normandy