Hi Weazle. The standard alloys on the mk6 GTD were the 17" diamond cut Seattle Shadows. and you're correct, Vancouvers we're the optional factory fit 18" option on the early mk6 GTD. This changed on the later models to the 18" Charleston shadows.
It is possible the original alloys were replaced with Seattles, or as Dubber36 has suggested, the diamond cut Seattle shadows may have been refurbished and powder-coated - especially if they were suffering from whiteworm corrosion.
Technically, for insurance purposes, if the original factory fit wheels have been replaced, this counts as a modification, and should be declared to your insurer. If they're genuine VW wheels that are the same size and offset, then I'd be surprised if there'd be any additional premium to pay.
I replaced my factory fit 18" Charleston Shadows with VW 18" Rotary's. I informed my insurer that they were exactly the same size and offset. I had to confirm that they were secured with locking wheel bolts, but there was no change in my premium; my insurer just noted on my policy that I had replaced the original wheels.