The github counter is misleading - it is counting auto-generated C headers used for calling assembly code. The repo also contains a libaom submodule, but the only parts that are used are initialization tables and the transforms.
The stats aren't wrong. If it isn't 'real code' then you shouldn't store it in your repository, you should autogenerate it as part of your build process.
The C part is mostly for low-level functions and was brought in to help bootstrap development (it's easier to work on improving a working encoder than one that doesn't work yet). The amount of Rust code is expected to increase a lot over time, while the amount of C code is expected to either decrease or remain constant.
80% of codebase is C. How is that Rust encoder? Seems like a binding to C AV1 encoder.