Once the OP boarded, and once all other medallions have boarded, any F seats were up for grabs. Yes, it might be a ncie gesture to UG an elite, but that is not the protocol on DL. Funny that you ask, since you already knew the rule very well since you mentioned it in your OP.
What do you mean - "any F seats were up for grabs." Certainly they're not up for grabs for anyone - a free for all for any open and vacant F seat????
All I am suggesting is that if boarding is complete and there is a seat open in F - then IF the FA chooses to come on board and upgrade someone, it should be an elite and not a flight attendant - seems like the obvious and most logical thing to do....