Does llama-index use KNN or HNSW for

Nope

Not unless the vector store you are using supports that

so I suppose that a nope for native hnsw. But if I am able to hook up Chroma to the engine, which uses HNSW by default, that should be the case right?

Yup exactly 👍

The default vector store is very simple -- just a numpy array of vectors and pairwise cosine similarity