Did I get it right? The crate lets you directly input data into a typst document from an arbitrary struct?

Well, typst provides those means in itself, this crate just makes it easier to fulfill the necessary trait.

Also out of scope of OP, is typst in anyway capable of typesetting music notation?

I don’t really think so. Music notation is also pretty specific, so I’d look into specialized software for that (although it might be theres a latex package, those things are everywhere ^^).