You didn't tho. Investing in gold should be a long-term investment, and in a long term price of gold trend is still rising. It was constantly rising till 2012, then fall a little bit in 2012-2016, and is rising again.
The point is that it is very difficult, practically impossible to time the market. So the commenter was right - he got lucky that one time.
And if you were trading in the markets for many years, you would see that occasionally you would be lucky too. But unless you employed some kind of risk and money management, the other times where you would be wrong (and those times would definitely happen as well) losses would cancel out all your lucky profits.
I was lucky a couple of times too and it means nothing.