You can see the same thing with high-quality JPEGs vs PNGs for photos. There's a decent size reduction, but to see the difference, I have to open the files in Photoshop, zoom in, and toggle which one is visible. If I compute the difference in Photoshop, it's usually only a few levels different, and never more than ~8. That's really hard to see.
Sadly, they're not even using a modern codec like Vorbis or Opus.
That said, while I doubt I can tell the difference between FLAC and high-quality Vorbis, once I learned to listen for distortions on the high end, it's a little hard to unhear. Same as looking for blocking artifacts in JPEGs or video.
Music is not white noise. It does not maximize the channel capacity of the audio spectrum. As a rule of thumb, FLAC compresses most anything a human might call "music" in about a 2:1 file size ratio.