You're right, my mistake. Thanks for catching that. I was looking at the first couple sentences (reading UP TO line 21 instead of starting there).
I'm going to edit my original post so that it doesn't say something dumb.

You said:
"high earners have incentives and means to avoid taxes under "progressive" systems - which is clearly a problem"
Is it? Why is it a problem? Is someone presenting that as a reason you shouldn't have a progressive tax system?
In context, it's presented as a way of showing how you might get the same results from the flat tax that you do from the much more complicated progressive tax.
Could we really feel good about saying Psg A was written in order to answer the question "What are some objections to progressive tax codes?" Doesn't Psg A feel way more centered on flat taxes? It seems more likely that Psg A was concerned with answering a question about flat taxes.