Sowell has written plenty of books on these topics which are worthy reading. The quote is from one of his books. The twitter account I linked is not actually Sowell's. He doesn't have twitter. The account simply posts quotes from his books.

So no, it's not just a bumpersticker retort. Growing up in a poor south asian country, I was taught to only care about the food on my plate and not be jealous of the food on someone else's plate. That made me work hard at putting more food on my own plate without being jealous and stealing from another's plate.