There seems to be quite a lot missing from the Chrome configuration too

It's a pretty simple project, and the license is MIT. Wouldn't be hard to let them know what's missing: https://github.com/corbindavenport/just-the-browser

Are all options available to the group policy? Since this is not directly modifying your app and merely creating a group policy for the browser to use, there might be some things not able to be set there. I have not experience with these group policies. Just thinking of why something might be missing as you stated. It could also be considered out of scope for the dev making this project.