I did check that page, as far as I can tell you still need to run Cargo which I don't want to do because I don't care about Rust.
I'm not complaining for the sake of complaining, I'm saying if they want to play in the Big Boy leagues, they need to do things right.
You do need Cargo to build from source.
If you're on Arch, gentoo, or openSUSE, you can use the package. It is true that Debian has not packaged jj yet.
It'll get there, and it's fine if you'd rather wait until things are more mature.