If for some strange reason you need to get the .proto files embedded in mach-o binaries using protobuf-c library, let's have a try with this script.
NB: this script does not work with fat binaries. Extract the binary for one architecture before using it.
python3 protobuf-c-extractor.py -i/--input <binary_file> -o/--output <path_to_dir>
Inside samples folder there are a few .proto files extracted from Telegram macOS application.