This is exciting -- Rust has already shown great results through its implementation in Firefox (made me dump Chrome for Quantum)[0], so building a whole new operating system using the language makes perfect sense -- a great test bed for the language, imo.
And given the fact that Haiku OS team's goal is to build a 'unified' and well-fused OS for personal use [1], using a single language should greatly help with reducing complexity and improving robustness.
Haiku is written in C++, as are most of its utilities I imagine. I don't see them redoing all that effort for such trivial gain. They're just talking about tooling and language bindings for people developing applications for Haiku.
You might be thinking of RedoxOS [0]?
[0] https://www.redox-os.org/