I'm sorry, but if they've told you that then it's my opinion they might just not have a clue what they're talking about. During the 18 month ownership of our TA which be bought new as a pre-reg with only 9 miles on the clock, we ran it exclusively on Shell V Power Nitro unleaded 99 octane with no ill effects. Over 13,500 miles, the car ran extremely smoothly. The engine didn't falter once, never missed a beat.
I would personally have asked to see the plugs they removed, but then you'd have been at the mercy of whether they were actually showing you the plugs they'd removed, essentially, you'd be taking their word for it and you're in no position as the customer to know otherwise. This was the main reason why I changed the plugs myself in our first 500, our old 1.2 POP. The dealership mentioned about changing them on a dealer service and I told them they didn't need to because I'd done it myself, even using genuine Fiat branded NGK plugs. I saved myself a small fortune doing that because the dealership charge for changing plugs was ridiculous. I cost me £16 doing it myself. Even if I'd changed the plugs in our old TA, it would only have cost me £20 at the time buying the plugs from a Fiat dealer off the 'bay.