The comment is not "By signing up for Quantopian, you pay a membership fee to Quantopian, which is run by large trading firms."
The comment is "Large trading firms make money by being the counterparty of day traders making mistakes." When you mess up - which isn't just losing USD, it is underperforming the market's usually-positive return - there is someone out there on the other side of the trade who is buying what you sold and selling what you bought. When you underperform the market, they overperform the market by exactly as much.
The site's owners decide to invest with the best algorithms on the site, and you get a portion of the returns if they choose yours.
If I trade profitably but making below market returns, the notional someone else who is taking the opposite sides of my trades is not outperforming the market by as much as I am underperforming, they are losing money and, thereby, underperforming even worse than I am.
There's obviously people in the market overperforming, but it's not someone taking the opposite of my positions that is doing it.
