9% of $9k is $810 so the total after two $9k contributions and one year of growth is $18,810 not $18,180.
The article also has a math error in that it cites 25 years of growth but the chart only goes out 24 years.
The S&P 500 has returned ~9% annualized on a long-term basis.
The article implies that you earn 9% annualized, long term, as well as 2% from dividends. Dividends are counted as part of total return, not because they provide total return but because it's far more complex to explain adjusted close for historical performance.
Their final portfolio value of approximately $722 thousand will generate passive income of $14.4 thousand annually from a very reasonable 2% portfolio dividend yield.
This is one of a long line of web articles that implies that dividends are income. They provide zero total return. Reinvested dividends enable compounding but they are not the cause of compound gains. Only share price appreciation provides portfolio growth.
At age 65, the investor will have a common stock portfolio of nearly $850 thousand, which generates income of approximately $16,800 completely passively with a portfolio dividend yield of 2% (very attainable). The investor can withdraw more if they are willing to reduce the capital base of their portfolio.
Again, the error: Receiving $16,800 of passive income from dividends means that the value of your equity positions decreases by $16,800, assuming no change in share price. Your portfolio value remains the same. If you withdraw that $16,800 then your capital base is reduced by that amount. Withdrawing more than $16,800 decreases your capital base even more.