I think this is miss-representing Buffett's position here. First off, he explicitly states that the riskiness of an asset should not be measured by its volatility:
> Assets can fluctuate greatly in price and not be risky as long as they are reasonably certain to deliver increased purchasing power over their holding period. And as we will see, a nonfluctuating asset can be laden with risk.
He goes on to describe what sorts of investments he thinks are worthwhile. Namely investments in assets that will continue to be productive in the long term.
> Investment in productive assets, whether businesses, farms, or real estate. Ideally, these assets should have the ability in inflationary times to deliver output that will retain its purchasing-power value while requiring a minimum of new capital investment.
Again, this isn't framed in terms of risk. Probably because with a diversified portfolio the risk is not at all analogous to flipping a coin (there is obviously still risk, but it's not that simple). Instead, it's framed as current versus future consumption.
As for gold, I don't think he frames it in terms of speculation versus investment at all. Buffett recognizes that gold will probably continue to hold value. Instead, he decides to frame the question in terms of whether gold is a productive asset. It isn't. It will just sit there.
> You can fondle the cube, but it will not respond.