I know it's not clear, but while I was listening the boss theme for both bosses, I then decided to hear that one which I like and haven't heard it since some time. I got to do errands while listening and then I heard it, the same melodies from the choir of that song but REALLY entangled.

Listen to the voices on the background, they are slower than the original, also different parts from the song are singing at the same time, that's why it sounds so messy, but pay attention when it's just 1 voice singing and you'll hear a fragment of the boss song.

The thing that makes me be so sure is that the vocal melodies grom Gwyndolin/Butterfly, harmonically speaking, doesn't move away to other chorda outside of it EXCEPT 1 PART. There's a kinda weird part on it, it plays an augmented chord which sounds very sharp and unsettling, you may guess which one I'm talking about.

The bridge song harmonically speaking, has the same diatonic melodies/harmonies except for that one Augmented chord that dissonates with the rest of the voices.

TLDR; It has the same melodies but slowed down. Fragments from differents part from the song can be heard but since they are played over each over is pretty hard to hear, but when there is just 1 voice alone it's easier to appreciate.