Exactly. In fact, I love people like the OP. Once they board, they lose their right to the UG. If still on the UG list, I am certain to ask the GA if the F cabin has "boarded" full and make it clear that I realize they already mentioned F has "checked in" full. More than once I have scored an UG in such cases by not boarding. In fact, I now carry the rules with me (Emailed to me by DL when I asked for clarification since a GA wasn't following the rules in ORD).
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.
+1
If you want the upgrade, don't board. I know many people wish for GA's that come on board and upgrade elites if there are empty seats, but this is not how the current policy works.