That's not exactly what Buffett means when he says "invest in things you understand". What he means is: what will the market in which this company is selling look like in 10-20 years? Can you be very specific about that? Can you give me an invariant that will very likely stay true for that period? If you can't then you don't understand the future of this company. And this does not mean that the future of the company will be bad, it may be excellent. Just that you specifically don't really know that.

This plays well with his love for almost trivially easy businesses, like Coca Cola. The simpler it is the easier it is to predict. "You want a business than can be run by an idiot, because one eventually will".

Buffett has a good understanding of trade relationships between the countries, he wrote a good piece on the US trade imbalance many years ago. He probably has an excellent understanding of the bond market. But he can't predict where will this go, so he doesn't use it as a basis for his investment decisions.