I've agreed with this sentiment for a while now. I think the actual $10 billion number is not important/up for debate, but the point is that at some point, a very wealthy person obtaining more wealth is almost definitely bad for society.
It's possible that in an economy, when the billionaires win, we all win, if the productivity gains are enjoyed by all. But as we're seeing in the US now, after some point in massive wealth, the wealthy inevitably use their economic power to buy political power, in order to influence laws that give them more economic power.
Not to mention if someone has $10 billion, it's much more likely they've done immoral or illegal acts to obtain that wealth than someone with $10,000. Money attracts power, and power corrupts.