For those of you devoted to decoding Buffett's every pronouncement, his meaning is clear: Bitcoin is not tangible investment. It don't generate earnings or pay dividends. How do you really value Bitcoin if it have no intrinsic or "book" value? And people who buy bitcoin are simply betting that the price will go up.
It generates money for miners, they creates Bitcoins and get paid for it. How do you value US dollar? How much cost paper?:)