Investing 90% of a portfolio in a large-cap US index fund and 10% in short-term treasuries is not considered a "conservative" asset allocation. It's missing a lot of low-hanging fruit in terms of portfolio diversification. For example:
The suggested portfolio isn't diversified with an International stock market fund. The S&P fund isn't exposed to small-cap and mid-caps, like Vanguard's Total Stock fund. The bond component is small and has no exposure to intermediate/long-term bonds or corporate bonds.
A more conservative portfolio would be, for example:
60% Total US Stock Market
20% Total US Bond Market
20% Total International Stock Market
Edit: I'm not saying that Buffett's suggested portfolio wouldn't outperform a conservative three-fund portfolio. Just that his 90/10 portfolio is very aggressive with a large-cap tilt.