So, this might be a case of me assuming too much, but I basically read every instance of _cosmetic filters_ basically as _css styles_. There are a couple CSS files in the git repository that have "filter" in their names[0], but yeah I can't actually be 100% certain at a glance that they're using CSS and not manipulating the DOM with JS.
[0]: https://github.com/gorhill/uBlock/tree/master/src/css