These stocks represent part of existing companies. If these companies do well the stockholder will/might receive extra dividends. So if you have stocks of a single company then you will make money in the long run if the company does well (and vice versa). Generally people do not trust a single company, so if you diversify (different strategies apply here) you can make your outcome dependent on a countries economy or the world economy (if the world economy does well, your portfolio will do the same)

Oh sorry, I wasn't precise enough. I meant specifically forex traders (because the article was only about forex traders). Since currencies are not cash-flow producing assets, I don't see how, on average, everybody can end in the money.

About stocks I absolutely agree - history tells us there's a long-term positive (US) market return so it's clear that an average investor should get exactly that return minus costs/fees.

This is a good point. One could argue that some people buy currency on forex to use it for a different business which gives them a gain. So your selling benifits them and you.

Long term non-trader money is also part of the market (stock buybacks, dividends).