The file's sha256sum can be verified before the file is sent to any users, so there's no chance of RCE there, even with a hypothetical C brotli-- but a reproducible decompression is key.
Additionally, if you want to do the decompression on client-side, facilities like SECCOMP simply may not be available on that platform. And in that case, having a language like Rust to guard against RCE is an excellent idea. Also it is easiest to maintain the same code running on all platforms rather than C code where SECCOMP is available and Rust code where it is not.
Is the brotli decompression step really the most dangerous vector on the client-side? What about all the non-verified client-side native code that actually interprets file data? From a practical perspective using the C code doesn't deteriorate existing conditions and using the Rust code doesn't improve them.
Additionally, if you want to do the decompression on client-side, facilities like SECCOMP simply may not be available on that platform. And in that case, having a language like Rust to guard against RCE is an excellent idea. Also it is easiest to maintain the same code running on all platforms rather than C code where SECCOMP is available and Rust code where it is not.