Highly doubt it's "fake". Dell gets their junk early all the time.
Regardless of when they got it 580s don't have Ellesmere chips.
Some 580s do have Ellesmere but revision E7. This is an Ellesmere revision C7 and yes is a 480.
This has already been asked several times in dell forums on why dell sold rebranded 480s as 580s.
Most likely because Dell had a lot of cards so they modified the bios to say 580. They can't sell new computers with an older card.
I doubt they put a real 580 bios on it as even hwinfo will say this is a 480.
Furthermore, the only AMD cards I ever have seen in gpu-z to display (TM) after Radeon are only fury and 400 series cards.
I haven't seen a single 500 card display the (TM) after Radeon.(besides dell ones of course)