Rather than having the LLM and human devs all guessing from verbose variable names, can't they both use a language server that observes the code and can surface that kind of structure info cheaply?

Part of the point of types is enforcing more of the contract at various code boundaries (function, module, etc), and that enforcement is specifically so that you don't have to keep the whole codebase in your head / context window in order to be able to work on it.