How is it related to SOAP in any way?

I'm not OP, but the main parallel is a well defined schema of communication between the services using different underlying technologies.

SOAP is in my experience really hard to use and get right, compared to protobufs that bring well understandable set of primitives and intuitive support in many languages. gRPC is a solid carrier for protobufs. Yes, gRPC has many cons (e.g. with undefined/nil values, etc.), but overall it has worked great for our usecases.

Yeah, one way I've describe gRPC to colleagues (which may help or hurt depending on the perspective) is that is "SOAP, but without all the lunacy"