> What is the difference...?
Remember when the price of a barrel of oil went negative?
That's the difference.
With something like BitCoin, there is no meaningful way that it could possibly drop below $0 in value, because there's nothing behind it. If everyone gets bored of digital currencies, it will become worthless, and nothing will change in physical reality[1].
Meanwhile if you buy oil futures, then you must take delivery of... you know... a bunch of crude oil! A fleet of tanker trucks will literally turn up at your doorstep and demand that you provide storage for your oil.
That's why the price of oil can go negative. The price is just a reflection of a reality. The oil that is being pumped out of the ground isn't just a number in a computer system, it's a physical liquid that has to go somewhere thanks to conservation of mass. If you buy it, part of that cost calculation is that you[2] will have to store the physical goods somewhere.
The price went negative because oil storage ran out, and the producers would pay people to take the oil off their hands. They had literally nowhere left to put it, and you can't just dump it in the ocean.[3]
So you see, Buffet saying that BitCoin is fundamentally worth $0 is in a way a worse indictment than oil going negative! It's saying there there is nothing behind it. Nothing of value that you might have to "deal with" as its owner.[4]
Even fiat like the US dollar has something behind it giving it inherent value: citizens of the United States are required by law to pay their taxes in USD. They must use it, whether they like it or not. They can't just walk away and stop using it, they'll go to jail if they do. Essentially its backing is the US government and associated institutions that use it as the preferred currency. BitCoin has no significant state support like this.
Now... you would think that BitCoin could get state support, but that's a guaranteed economic catastrophe for any large economy that switches to BTC for their official currency. Partly because that government would lose control over their financial system, and more importantly because inherently deflationary currencies like BitCoin are certain to cause economic deadlock where nobody wants to spend money "now" because they expect their future spending power to be even greater.
[1] In terms of products and services. I'm sure a lot of people will cry, but that's a secondary effect.
[2] Or whomever you sell the future to.
[3] Well, you can, but it's illegal these days. You can be assured that they would dump it without a moments hesitation if it wasn't illegal, because it would "make the line go up" in the futures chart.
[4] There are lots of things like this. Property values can go negative if the taxes and legally-mandated maintenance cost exceeds the expected income.