They are using RAG. Grep is also just RAG. The better question is why they don’t use a vector database and honestly the answer is that these things are incredibly hard to keep in sync. And if they are not perfectly in sync, the utility drops quickly.

By RAG they mean vector search. They're calling grep "agentic search".

I forgot to mention that Aider uses a tree sitter approach. It's blazing fast and great for quick changes. It's not the greatest agent for doing heavy edits but I don't it has to do with them not using grep.