Yes, you can get upgrades when standing by; we've done it. But the part about being behind the current pax is true, within tier. If you were told that you were after all pax, including GLD, that is not correct. When we did this, being EXP placed us at the top. There were no others requesting upgrades out of MCO, so it was moot, though.
Cheers.
Not doubting, but are you sure of the rule? On UA, it is different. You MUST be confirmed on the flight in Y before you can even get on the list for an upgrade. So that means that all confirmed pax on that flight will get the upgraded seat before standbys, if the flight is full.