Of course "it depends upon what you invested in" but your parent comment is referring to the market average or equivalently the return of an index fund. That's

Of course "it depends upon what you invested in" but your parent comment is referring to the market average or equivalently the return of an index fund. That's the only way it makes sense to compare the returns of different time periods. After all, in any period there are stocks that offer large returns but that of course doesn't make the stock market a sure bet since we only know the good stocks in hindsight.