There are node modules already that call into WASM-wrapped rust. (One of the most common source map generation packages does exactly that and it is very heavily installed by a lot of frameworks and packagers today.)
I don't think prettier is intending to do that in this case, they seem to want to continue to compete as a pure JS implementation but having a "worst case, we WASM wrap biome as the future of prettier" possibility opens up future opportunities and pushes "internal" competition.
Are they planning to run Rust in the browser? Or make some sort of node module that calls down into rust?