My mk6 GTD had the winter pack and it definitely took 5 litres. I know this as I use a 3 litre squash bottle to fill it. Maybe yours is lying about it being empty. Or they never fit the larger bottle with xenon only. It's mentioned in the winter pack extras.
I had xenons on my mk6 GTD but I didn't have the winter pack. I never consciously measured the volume to fill it when the warning lamp came on, but it did take much more than 2.5 - 3 litres.