I don't think scenic is written in Rust. A big chunk of the source code in the topaz layer (the view layer) is still written in dart. With that said, many were actually surprised when it was revealed that ermine (the new fuchsia shell) was written in dart/Flutter [1] since earlier code for it was in Rust [2]. Guess that didn't pan out.
[1] https://fuchsia.googlesource.com/topaz/+/refs/heads/master/s...
[2] https://fuchsia-review.googlesource.com/c/topaz/+/184430