Indeed, as you’ve found, the main @logseq/rsapi is just a wrapper for platform-specific code (see index.js), which is a binary blob. It’s only “open-source” in the most literal sense, in what is required to build the repo locally.
I’m actually about to push a big update to my repo, I’ve figured out lots of details and endpoints and workings and documented a bunch of them. I believe the rsapi package is actually Rust code compiled to WASM.