No, just download the source and check. It's 14M C/C++ and 4M in Rust. There is another 3.3M in Go and 1.1 in Dart. This is a usual trope that Rust is just about to replace C++ but in fact more and more gets written in C++.
A lot of what you're seeing is third party dependencies, not code fuchsia developers have written. A lot of that is third party code is also dead and isn't compiled into anything (like mesa). If you do a more detailed analysis of what ends up in an actual fuchsia image, it'll look a bit more like 50/50 (and that's close to what you get if you just remove the third_party/ directory). It would be strange on fuchsia to start on a new project and choose c++ over rust these days. Most critical components of the os including filesystems, network stack, linux emulation layer, init system, etc are all rust. Things which are not are likely to be rewritten in rust eventually.
Go and dart are basically gone from the system as well. Go is only used in the legacy netstack which has been displaced by one written in rust, and is otherwise only used in host tools for building fuchsia. I believe dart is almost gone, with a few remnants left in the form of build tools. Flutter does support fuchsia (and that is dart), but that support is not maintained in a fuchsia repo.
curl -s "https://fuchsia.googlesource.com/fuchsia/+/HEAD/scripts/boot..." | base64 --decode | bash