Any discussion of middle class should start by defining middle class. Otherwise, it always seems pointless since no one is talking about the same thing.
> In our analysis, “middle-income” Americans are adults whose annual household income is two-thirds to double the national median, after incomes have been adjusted for household size.
And they seem to use "middle-income" and "middle-class" interchangeably.
Note that this definition is quite different than what may have been used in your history/economics classes in school/college.
I’d venture a guess that the concept (“Middle Class of the Creator Economy”) was thought of before any backing was found.