Since it says "voluntarily", it definitely shouldn't apply to cancellations, misconnects, or IDBs. The question is, is arriving at the airport late (flat tire rule) voluntary? I'd hope UA would continue to allow free standby if you show up within a certain timeframe after your scheduled flight, even if it does leave a window for abuse by people booking a cheaper earlier flight and then showing up for the flight they want and claiming a flat tire.