Well, it could be reworded with less density, but actually it is accurate (or it would have been something like check whether the string contains only letters and is an isogram). Atention to detail is important in programming ;)

Thanks for the response! It just feels like it is a bit too open to interpretation.

You misread the description. We're given a string that contains only letters, there is no need to check, we must only check it does not contain duplicates.

This kata is asking to "Implement a function that determines whether a string that contains only letters is an isogram."
Most solutions don't check for digits. And there are no test cases for that either. What am I missing?