His alternative to using BTC is using fiat money, which also has no intrinsic value. So calling out Bitcoin specifically for having no intrinsic value is unnecessary. He would have to clarify why exactly he believes that fiat money (the alternative) has any intrinsic value either.
What people also tend to forget is that USD for example is like 90% "crypto" currency as well. Only a small portion of USD is printed or minted, but 90%-95% (can't remember now the exact number) is in digital form. The difference is that central banks can create USD (and other fiats) digitally by pressing a button. But in order to create more Bitcoin you have to use quite a bit of energy to mine it, as it is also criticized by many.
In practice one might say USD has even less value than BTC, when you think of the fact how it's created. However, it has value because the government says it does. One day very soon the whole crypto vs fiat discussion will be meaningless. Governments will win the war-on-cash eventually moving all fiat to digital form as well. By then probably each country will have their own cryptos as well.
I've read a lot of mr. Buffett and I can see where he's coming from.
He has also said many times he doesn't invest into things he doesn't understand. He doesn't master every single thing in the world and I doubt he has any interest to master BTC either. You only see the value in Bitcoin if you understand the bigger picture. Hence, it's easy to see why Buffett (or any investor like that) will make such comments.
I'm also pretty sure that soon every top level investor is one way or another invested in cryptos. Maybe they own them directly in secret or their wife owns them or they own shares in some funds etc. I find it very hard to believe that any greedy investors in this world would bypass this opportunity. I just don't buy that.