The explicit text from UA
can be found here.
Waitlisting for travel awards is permitted on flights operated by United, and will be automatically confirmed, if the waitlisted cabin is available, based on the status of the account holder. Courtesy waitlists may be requested and held up to 24 hours before itinerary departure. Confirming a Standard Award and waitlisting for a Saver Award is not permitted.
It is two bullet points above the text in the OP.