It really is a game of semantics as another person commenting mentioned. It might feel like there's something distinct here but all RPC was remote procedure cal

It really is a game of semantics as another person commenting mentioned. It might feel like there's something distinct here but all RPC was remote procedure calls aka functions and produced tightly coupled clients and server code. And the term API is not specifically owned by HTTP calls or REST/GraphQL, it's a definition of an interface for consumer some service. Even in your case whether you feel like you see it or not, every function is part of an API.