Did you read their comment? He assumed you did the opposite of buying low: "Let's assume you bought SPY (very conservative ETF index) right before the crash at its high of 156.33 (Oct. 12th 2007)" Note the "at its high". Even if you miraculously had horrible luck and bought into an index at literally the worst possible time, you would have still made money.
This holds for practically any index out there. I personally hold VFINX. These are the returns for VFINX if you bought at the yearly highs and the yearly lows, per year:
2006 | 1.95x | 1.70x
2007 | 1.53x | 1.72x
2008 | 2.82x | 1.67x
2009 | 3.50x | 2.16x
2010 | 2.34x | 1.90x
2011 | 2.12x | 1.79x
2012 | 1.87x | 1.65x
2013 | 1.65x | 1.31x
2014 | 1.34x | 1.16x
2015 | 1.23x | 1.13x
2016 | 1.28x | 1.10x
2017 | 1.07x | 1.00x
That's right: the far column is the absolute worst case scenario, what would happen if you were miraculously horribly bad at choosing when to invest. Even in the worst case scenario, investing at the worst point in the worst year, $90 would have turned into $153. Investing at the best time would have given you $315! As long as you don't instantly sell the fund you'll come out on top.
This isn't rocket science, it's not some complicated stock pick, it's not hard to buy, and it's backed up by a hell of a history: index funds with low expenses give you a good return, no matter who you are. I know one person with $400 in VFINX and I know one person with several million in it.
And if you bought in 2007, before the recession, at the absolute worst possible time, you would still have nearly doubled your money in the last ten years - a 1.72x return.
Warren Buffet put a bet on this ten years ago, against a series of hedge funds, and as of now, with just months to go, he's winning. Not "winning against the worst", not "winning against the average"... the index fund - VFIAX (the Admiral class of VFINX) is beating every single fund handily, even though the bet started at a time advantageous to the hedge fund.
If you, or anyone else could, they would be Trillionaires.
By picking an arbitrary point in time - and comparing it to another arbitrary point in time (say, 'today'), you create a straw-man argument.
"Even if you miraculously had horrible luck and bought into an index at literally the worst possible time, you would have still made money."
Yes, stocks went from some low point, to a higher point today.
When you say 'this holds true for every index' - well, a broad ranges of indexes roughly encompasses the entire market.
Ergo - you're really just investing in the stock market, not indexes.
Unless you know something very specific about VFINX - and have research that other people do not have on the constituent companies, then you are just throwing darts at a wall. If you made money, great, if not, then it's the same thing.
What someone who doesn't have specific information is doing when they play the market willy nilly, is simply riding the overall market. And like other asset classes, it goes up and down.
There are historical averages for markets, and in the long run you can expect to earn just a little under that.
Interest rates are at an historic low, and we have bubble-like conditions in the market - if the Fed increases by a couple of basis points, much of that gain will go away.
Do you understand the fallacy in that logic?