To be fair the site claims they have an open source application yet there is no link to the source code.

Another commenter mentioned github so I searched around & found a potential github project but they're at 4.0.0 release while this site you linked claims to offer a "4.2.0.apk" so I guess it's some random 4.2.0 release version?

Are you able to link the source code that this website refers to?