The existing mechanism to get QUERY semantics is a POST that encodes the “fetch parameters” in the body and the response contains the fetched values. You then out-of-band document that this specific use of a fetching POST is idempotent.

This is literally expressed in the document in section 1: Introduction. They just want to take that POST request and replace the word POST with QUERY which also means the server is intended to assure the request is idempotent instead of needing that documented out-of-band.