Thanks for the question! 😊
The uniqueness of icons is one of the criteria we follow at Iconfinder for deciding which icons get approved.
In the article you shared, we are referring to icons that differentiate themselves (be it with their style or concepts). So, it's more like Unique vs. What everybody else draws.
In general, icons that use the same concepts and the same style that everyone else's icons have little chances to sell well. So, if you create the typical "Business" set in outline style (with basic shapes, nothing special), the set will just get lost in the mass of business sets that all look the same.
If, on the contrary, you try to make the icons more interesting (be it by adding your personal style elements, playing with the drawing style, with the colours, theme, concepts) as described in the article, customers will notice them. And to be noticed is really important in a competitive market such as the icon market.
We don't have exact numbers for the sales because the uniqueness is rather subjective.