Use bundles.configuration.location in admin ng settings.yml

Is this settings.yml file being written to etc/index/adminui/settings.yml by the bundle when it's instantiated so that it isn't really editable at that location by someone?

No, it is always there. This is just about having a sensible default configuration.
Have a look at the corresponding pull request:
https://bitbucket.org/opencast-community/matterhorn/pull-request/480