A basic definition of trade is "to give in exchange for something else." Williams is not claiming that these individuals have engaged in charity. Rather, he is making the point that should be absolutely obvious. If I receive money for nothing, then it may be legitimate to say that I should "give something back." But these individuals already "gave" to society in the form of their contributions to commerce.
If you give a gift, you don't necessarily expect to get anything in return. If you sell a television, you expect money in return. The individuals sold their products to society for their own gain, not for the sake of the public. Yes, their products were beneficial, but that doesn't mean they gave them away. Don't get me wrong - I don't hold anything against them, and I don't think they necessarily owe anyone anything, but to call it "giving" when they were doing it for money is absurd.
"Giving back" may imply they owe something to society, but it doesn't imply they didn't earn their wealth. I would agree it's a bad way to put that those with wealth should give to charity, but again I don't call selling a product giving in the sense that "giving back to society" implies because it's not.
