Not true. You can add yourself to the standby list during OLCI, and the boarding pass will print out with your original flight info, and a second flight listed saying "standby".

(I've done this a number of times going SEA-Greater LA. Matter of fact, did it last week- stood by for an earlier flight during OLCI, printed out the BP, showed up for earlier flight, was at the top of the standby list without talking to anyone at the airport, then exchanged the BP when called. Gave up F for an exit row, but well worth getting there earlier.)