I think he means 90% of retail punters lose money - in this you can also include people who work in prop shops.
Zero sum game depends on the time horizon, instruments traded, and strategy.
High frequency trading in the stock market is nearly a zero sum game. On the other hand long term investment in the stock market is not a zero sum game.
Foreign exchange is a pure zero sum game that is partaken by pretty much every individual on earth.
A lot of derivatives trading is zero sum. However for both FX and derivatives you need to take into account the fact that hedging of physical/financial products (that might not be zero sum) constitute a major part of the market.
As for the question about whether most traders win or lose, I would agree that most traders that make directional/proprietary bets (even at bank desks) are likely to lose. Flow is an incredible edge. The reason why many banks have had incredible trading revenues for the last year and a quarter is directly due to flow. Number of participants was low so spreads and volume increased per participant dramatically. Prop is a significantly different game.
Can someone explain the term 'market maker'?
Do any of you know if there are trading teams in corporate banking divisions? My question might make no sense but I was wondering if they don't need it for some clients?