It might be worth to continue reading at least up to the point where they explain why they chose rust, what benefits does it bring compared to the previous tool(based on JVM), etc. In a company like Meta, while inefficiencies are bound to exist, when it come to projects like this, where it's very easy to measure before and after, decisions to rewrite a huge piece of infra are not taken on a whim or because "rewrite it in Rust".