I am pretty sure you are misunderstanding the comment.
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.
Okay... but on Quantopian you just write algorithms, you don't have to invest your own money. You don't mess up and lose USD, you don't pay a membership fee.
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.
> 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.
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.
Counter position is hard to define in a large market, so take a market with two stocks A and B. You buy shares of A from counter-position-inc who moves that money into B (possibly buying the shares you just dumped).
Over a time period A goes up 5% B goes up 10%. You sell your shares in A (profitably) but under performed the average market returns by 2.5%. counter-position-inc sells its shares in B, over performing the market average by 2.5%.
