That is demonstrably not true. 1968 was a positive year!
1968: +11%.
1969: -8.6%.
1970: +3.6%.
1971: +14.5%.
1972: +19.2%.
$1 of S&P500 in 1968 would be worth $1.23 in 1978, 10 years later.
There are large drops in some of the years, but they are usually followed by outsized rallies in the years afterwards.
Source: Moneychimp’s CAGR of the Stock Market calculator. http://www.moneychimp.com/features/market_cagr.htm
Inflation adjusted were the key words.
Even your link admits poor performance during periods of high inflation, and gives several other example periods.
But here was my original source:
http://www.multpl.com/inflation-adjusted-s-p-500
With your link, if you're just talking about value of the stocks and controlling for inflation, one dollar of stocks in 1969 grows to only $1.03 in December 1991.
The S&P has not always generated positive returns relative to inflation.
Sometimes there are 30 year long dips.