He's just trolling.

Nah, it's a reasonable thing to look at, always remembering that no single pair of dates is going to be very informative.

Looking at the longer term, my estimate is that the S&P 500 is about the same valuation level as it was around the start of May 1998, based on smoothed real earnings yield, so the rate of return is presumably equal to the rate of value generation and there is no need to speculate on whether the market is overvalued these days. That return turns out to be spot on 10.0%/year, before inflation. It's 7.25%/year after inflation, a good stretch overall. (could be an overestimate because the recent upswing in net margins may not be entirely sustainable, but let that pass...)

Berkshire's total return is the same as that of SPY since the start of June 1998, almost the same date, so almost exactly the same rate of return.

So, same return for both for this pair of dates, the S&P is neither cheaper nor more expensive, but Berkshire is a fair bit cheaper that it was then. P/B 1.48 now, 2.52 back then 29.26 years ago. The valuation slide is probably a bit more than the -1.8%/year that implies, because book per share was probably a slightly optimistic metric of value in 1998.

Jim