I think something like this: Imagine a tool for querying remote grpc endpoints - you would point at some endpoint to query it / explore it, and the tool would request the proto schema files from a potentially untrusted source - something you would not expect to be that unsafe and trigger an rce.
probably via normal npm injection server side.
But once you can make people download your malicious js code using npm, why would you then need to inject malicious js code in protobuf?
Well, in this attack, you're using the vulenerable dev to modify their code to run a protobuf schema that's vulnerable; so then it can inject that vulnerability to the client code, and then you're exfilitrating 10's of users (the dev who ran this code isnt very popular).