It also says to start with a fully charged battery. I assume that's because it runs the battery down slowly and then shuts down while it charges. It does not give any indication of how long you can run it on USB. I've not tried to determine that.
I might try and see but I'd have to change the settings to keep the camera on. My battery is dead right now and my spare is out in the studio. The camera will not operate from USB with a dead battery, I just checked.
After thinking about this, I realized that the camera starts charging when USB is connected as long as the battery is about 75% charged. A full charge keeps the camera from going into the charge mode when powered from USB. I think I'll try and see tomorrow, I'll leave it on all day if possible and see how much the battery discharges if any.