I think your last point captures it, for various reasons (RL, inherent structure of code) iterative grepping is unreasonably effective. Interestingly Cursor does use embedding vectors for codebase indexing:

https://cursor.com/docs/context/codebase-indexing

Seems like sometimes Cursor has a better understanding of the vibe of my codebase than Claude code, maybe this is part of it. Or maybe it’s just really marginally important in codebase indexing. Vector dbs still have a huge benefit in less verifiable domains.