Thoughts on Jeremy Siegel and stock only investments for 15+ year timeline?

I was presented a newsletter with the following passage, given the current volatility in the market.

"Wharton professor, Jeremy Siegel, analyzed stock and bond returns in his classic book, "Stocks for the Long Run" (6th edition 2022). His research goes all the way back to 1802 (the very long run indeed) and concludes, not surprisingly, that over one and two-year periods stocks are much riskier than bonds. But across every 5-vear holding period since 1802, the worst performance for stocks, at -11.9%, was only slightly worse than the worst performance for bonds, -10.1%. (Incidentally, the best rolling 5-year period for stocks was +27.3% per year whereas the best for bonds was 17.7%/ yr). For every 10-vear holding period, again going back to 1802, the worst stock performance, -4%/yr, has actually been better than the worst bond performance, -5.4%/yr. (The best rolling 10-year period for stocks was +16.8%/yr; the best for bonds, 12.4%/yr). And across every 20-vear holding period stock returns were positive every single time, the worst being growth per year of +1.0%. The worst 20-year period for bonds was -3.1/yr. (The best rolling 20-year period for stocks was +12.6%/yr, and the best for bonds was 8.8%). Making these figures look even better is the fact that all of these percentages are real returns after inflation.

Siegel sums it up as follows (our emphasis added): "The probability that bonds and savings accounts have frequently outperformed stocks in the short run is the primary reason why it is so difficult for many investors to stay in stocks. But it is very significant that in the more than two-century history of financial returns, stocks, in contrast to bonds, have never delivered a negative real return over periods as short as 17 years. Although it might appear to be riskier to accumulate wealth in stocks rather than in bonds over long periods of time, for the preservation of purchasing power, precisely the opposite is true: the safest long-term investment has clearly been a diversified portfolio of equities."