That's a good point Ursula and it could well be that there are many of these species that could do well in our climate
It could well be that what I have read was an oversimplification, I haven't tried it myself.
In my garden A. aurea never gets any winter protection, but I live just about as far south you can get in Sweden. I think the hardiness has to do with the roots. They tend to go down to frost free depth as they age. Young plants are a bit more sensitive. However it can actually be grown quite far north as well, given some extra protection during the winter and a well draining soil.
Perennials and bulbs are fun like that, sometimes they can grow and adapt to very different climates than their origin. As you know it works the other way around as well, as sometimes cold climate perennials and bulbs grow very well in warmer climates.
Much worse situation trying to grow shrubs or threes out of zone here. Or perennials that need dry winter rest or intense summer heat to mature fully before winter and they can not do that in our usually cold summers, resulting in poor winter hardiness.
