EMSCRIPTEN_FETCH_REPLACE needed for loads to memory?

The testcase here:

https://github.com/emscripten-core/emscripten/compare/fetch-wasm-pthread-async?expand=1

passes as is, but if `EMSCRIPTEN_FETCH_REPLACE` is removed then it fails. That seems odd since the docs say

> // If the file already exists in IndexedDB, the old file will be deleted and a new download is started.

But the test does not do anything with IndexedDB, it just fetches to memory. Are the docs out of date, or does REPLACE mean something here?

cc @juj