Guess you missed it. It was stated more than once in the ATL DO thread. I don't have the time or desire to find it now, but it has been oft-repeated on the forum. Sorry you missed it.
What was stated on the ATL DO thread, which was a quote from DL management, is what I said earlier. You are correct, you don't lose the chance for the UG, but the one(s) who haven't boarded yet would be ahead of you in the UG priority. So, as it was explained, it is not in the GA's hands at all.
Honestly, it doesn't matter which way they do it (though since they stated one system, I believe it), JUST BE CONSISTENT. But I know that is a difficult concept for Delta to comprehend.
I was at the DO as well, and it wasn't declared (that I recall) anywhere in the QA or in informal gatherings. What WAS interesting was the "Gate Agent for a day" thing in which some people had differing results:
Canarsie saying if you don't show up, you lose your U/G spot (understandable) and that if you board, you lose your chance:
http://www.flyertalk.com/forum/delta...l#post14788979
But then a DL GA states otherwise:
http://www.flyertalk.com/forum/delta...l#post14876576
I've noticed in-plane upgrades happening more often than they used to, so I think there's been a focus on improving service levels here.
In any rate, the experience the OP had seems like a case of the GA doing the right thing.