Billionaires are not going to get a lot of sympathy, but it’s repugnant to characterize entire groups of people with the same brush.
The problem with this kind of rhetoric is that it overplays your hand, and therefore ends up detracting from your core points. Most of us want wealth inequality lessened. Demonizing one group over another is not going to get us there.
No, it's not. This isn't about whatever imagined prejudice or discrimination you're talking about. As the synopsis makes clear, the piece is marking an argument about the how "There is no way to be a billionaire in America without taking advantage of a system predicated on cruelty." Warran Buffett is being attacked because he has been able to manipulate the American system of capital, which the piece suggests is fundamentally unjust, to his extreme advantage, while millions of Americans are pushed further and further to the margins.
You can disagree with that premise, but don't pretend that this is discrimination.
The whole reason the article focusses on Buffet is to explore exactly your worry that there might be any such thing as a mixed group of varying innocence being painted with a single brush unfairly.
Pick the best possible example and judge, does the best known example of a billionaire validate the idea of billionaires? And at least this analysis decides no it does not. You may argue that this analysis is in error but I'm addressing your question about the right to do the analysis at all.
Billionaires are artificial constructs. The entire class can be judged and un-created with no moral problem at all. One's billionair-ism isn't like one's atheism or albinism. We can decide any time we want that no such thing as a billionaire should be possible, and make it so.
"Myth of the good arms dealer" seems fair. "Myth of the good mob boss" seems fair. The article makes the case that there's no way to accumulate $1,000,000,000 without taking advantage of people in inexcusable ways, so I'm not sure I see your criticism as valid.