It links with the system media kit libraries for input, which are backed by ffmpeg. For export it links directly to ffmpeg since the ffmpeg API exposes a lot more options and codec variations compared to BeAPI (ie. you can access the very strict/strict/normal/experimental/unofficial codec combinations).

Ffmpeg is a command line media editor. So is this just some graphical frontend?

ffmpeg is C libraries, the "ffmpeg" program just calls I to them but for instance they're used in chrome and many other software for video playback

Yes, but video playback and conversion is hardly the only thing they do

It targets Haiku, so it probably just uses the system's media libraries (Haiku does this really well)