That statement is correct when you are on PNR waitlist. However, waitlists on PNRs are purged a few hours prior to flight, if not confirmed. When you check-in, the agent needs to manually put you on the airport standby list (DM list). The GA will process DM list to clear standby upgrades. PNR has no bearing on DM list. Don't forget that a good number of passengers on the DM list may not have PNRs to begin with; interline passengers without reservation, employees, misconnects, etc. On the DM list, the priority is set by BP (Boarding Priority) code, than your status (UGS, 1K, 1P etc). Each one will be on his/her own and even a UGS may not get on if there is someone with a higher BP code, such as FFCC etc.