It's an RPC plugin but doing it out-of-process is weird (I need an RPC client to talk to my RPC client?). If you don't need a trust boundary or separate ulimits, loopback network traffic and context switches and reserialization are a really expensive way to mitigate a language having poor FFI plugin support.
If it were in-kernel and supported scatter-gather and zero-copy, that might be different (though some people even avoid going through the kernel).
The "RPC" that the proxy in Linkerd is doing is radically different and more expressive than the "RPC" that is running between the Linkerd proxy and your application, which is the whole point of the architecture.
If it were in-kernel and supported scatter-gather and zero-copy, that might be different (though some people even avoid going through the kernel).