One potential use case for a GC library in Rust is to integrate with a platform GC. For example, the WebAssembly platform will likely get native access to the GC of the associated JavaScript engine. This could be very impactful on the ability of Rust libraries to integrate with other languages running on that platform - not only JavaScript but also other GC'd languages compiled to wasm.
However, the current WebAssembly GC proposal does not allow direct references into the GC the way that shifgrethor does, because it is intended to support copying collectors that do not have pinning. This means you couldn't use shifgrethor's API on top of that proposal. It's unclear to me how all of this will shake out, but shifgrethor is just trying to research the implications on GC APIs if we have this particular constraint (that we support direct references).