> Something that surprised me is that [1] is of a minor key, but the solfege marks are for the relative major

Major and minor keys use the same shapes / solfege system. The major scale is "fa, so, la, fa, so, la, mi, fa", while the minor scale uses the same syllables but starts on la. This means that the shapes always represent the same intervals (i.e. pattern of whole and half steps) whether a song is in a major or minor. It's hard to describe but makes sense when singing.

I'm aware that they are the same intervals; perhaps my brain is destroyed by my music theory classes, but I really think of a minor scale as a different beast from a major scale starting on the submediant. [1] indicates that other movable do systems may use "la" based minors, and that it is "sometimes preferred in choral music, especially with children," so I suspect this distinction is something I learned.

1: https://en.wikipedia.org/wiki/Solf%C3%A8ge#Minor