Frankly, this is just panglossianism. Meaningless garbage. I like Buffett, but when he says that particular line, he's _______.
I can, if I have to, dig up quotes saying the same thing about the Russian stock market a few years before the Bolshevik Revolution, and you all know what happened there.
It's even easier to dig up quotes saying this from July of 1929 in the US.
I do think that the main risk to the US stock market is something *truly* big: a serious war, a Great Depression, revolution, etc. A bet on the US stock market in general is a bet on the US economy in general. That bet is NOT guaranteed, any more than a bet on any other economy is guaranteed.
Having read the whole letter, he spends an absurd number of paragraphs on a panglossian paean to the American economic system, not having noticed that it's actually been changed more than once and that the Reaganite system doesn't really resemble the system which created most of the wealth for the country.
That said, if you're hedging against the risks which will destroy the American economy, you probably still want to be invested in stocks and businesses of some sort as opposed to bonds or cash... just maybe not necessarily *American* stocks. Stocks represent productive business, and over the long run, productive business tends to be a better investment than other stuff. His letter is mostly arguing for stocks vs. cash/bonds, so I think he wasn't really addressing the US stocks vs. Chinese stocks question.
Reading the letter further, he makes a good point that the market is awash in capital -- but unlike him, I am aware of multiple enticing projects which have died for lack of capital. Seriously. They happen to be the sort Buffett doesn't like because he doesn't understand them. There's a sense in which the available capital is too risk-averse in a particular way. This is a side-effect of overconcentration of wealth; it's documented that richer familes become *more* risk-averse with their investments (unexpected though that may be).
Buffett's panglossianism about the US Economy is a bit of a sideshow. When you get into the details, you realize that basically he's running an insurance company, and he's running an *extremely well run* insurance company which would do very well under anything short of expropriation.
It's fascinating to me that Buffett spends two pages detailing what I have called "Nathanael's First Law of Investment Advice" -- if an investment manager can beat the market, the fees they charge will (with very rare exceptions) eat up all the gains and will mean that you can't *pay* them to beat the market *for* you.