Answer: I am inclined to be as disbelieving as you are. I suspect your customers may have seen ads such as the following at
or discussions such as this one
and assumed this meant winter hardy when grown outdoors in the ground. Careful reading qualifies it to being in containers brought into a sheltered place for the winter.
As far as I know, the hardiest are still those developed at the US National Arboretum. Most nurseries list the hardiest as being reliable to zone 6, a few will stretch it to zone 5 for selected among these varieties. And, this may be root hardy rather than completely stem hardy. (The Monrovia listing shows zone 6 at the coldest for selected crepe myrtles.)
Although we would all probably like to see some hardier crepe myrtles, I hope this helps.
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