Oh. This should really be clearer on these pages. I was using multirust-rs after having used rust up and had no idea this was related/a successor.
In the beginning, there was multirust. This was a bash script, or rather, a set of them, I guess. It didn't really work on Windows, though, being a bash script.
Then came multirust-rs. We already have a programming language that we like to write that supports the platforms Rust supports: Rust! So, port multirust to Rust.
Then, it was decided that mutlirust-rs would be the right path to move forward generally, so a plan was formed to figure out just exactly how we'd want such a tool to truly work, rather than just copying what multirust did, and so this became rustup.
Well, in the beginning, there was rustup, but a different rustup that didn't manage side-by-side installs :P