Depends which definition of RAG you're talking about.
RAG was originally about adding extra information to the context so that an LLM could answer questions that needed that extra context.
On that basis I guess you could call skills a form of RAG, but honestly at that point the entire field of "context engineering" can be classified as RAG too.
Maybe RAG as a term is obsolete now, since it really just describes how we use LLMs in 2025.