I took a deeper dive in Reticulum-rs. It is std. It implements 20% of Reticulum functionality. And it has 2 major protocol incompatibilities (like a different size for the MTU / Maximum Transfer Unit).
It looks like a quick vibe coded hack to implement a subset, tailored only for Beechat's own devices.
If someone would want to implement a full no_std Reticulum lib, they would need to start from scratch.