The damage is typical of the result of part filling with petrol or with contaminated diesel lacking the necessary lubricity. What the dealer means is that the swarf came from the high pressure fuel pump because there was insufficient lubricity in the fuel.

To take action agains the fuel supplier you need to be able to prove that you paid for diesel and either got contaminated diesel or petrol. If your receipts clearly show you bought diesel, then I think the dealer's advice is correct. But if you ran the tank low I think you are in trouble because then any lighter fuel floating on top of the diesel would have got into the system, and you could have misfueled at any time in the past six months.