I think his point is that the market value of Bitcoin is driven by speculation on its future value rather than by productive activity, and that that would make the gains much less guaranteed than gains in investments associated with something that provides value to people.

To be clear my own position is less extreme, though similar. I'm more in the "Some people may like it as a currency but if it's a currency, it's a lousy one (high volatility), and if it's an asset class it's pure unregulated speculation" camp.

I do have the persistent suspicion that there's something I'm missing, because how could so many people like something I think is pretty silly if there weren't some kind of substance to it, but it seems like the operative question is "what is the value of Bitcoin?" not "Would Buffet really turn it down?"