The conformance suite tests encode and decode for single messages; you wouldn’t run it for a custom framing implementation; The gRPC interop tests are closer to what you’d want.
Framing appears higher up the stack, as an RPC transport, or structured storage like the recordio format referenced by the author. The article sounds like the client expected application/protobuf but the server sent application/custom-protobuf-framing.
I think it is easy to confuse the serialization format from framing because many serialization formats do self-delimit. And the protobuf terminology of "message" may suggest framing...
Framing appears higher up the stack, as an RPC transport, or structured storage like the recordio format referenced by the author. The article sounds like the client expected application/protobuf but the server sent application/custom-protobuf-framing.