Terrible article.

The stock market doesn't compound. Stock prices go up and down at unspecific percentages. When someone says the stock market goes up by 10% per year, what they're really saying is "looking at historical trends of the market, we see that a $100 investment over the last 30 years would yield a current value of $1745 today. We can then use a compound interest formula to equate that to 10% growth per year, compounded annually".

We're choosing a formula and plugging in numbers to find a percentage that makes sense to us. The market isn't actually growing through compounding formulas, that's just what we're using to describe the growth.