The way things are defined in the US, the BMW 5 series is a luxury car. It represents close to the maximum amount of amenities for a given size class and, well, the car magazines call it a luxury car. There's not much else to it...
It's certainly possible they're equipped differently here, as so many cars are. If that's not the case and Austrians simply have a different sense of "luxury car" I wonder what you guys have that trumps the 5 series in that size category, as far as "luxury" is concerned.
BMW is an expensive brand, but the 5 series is mid-range.