The thrust of the point is that while the wealth of a group will rise on average when playing a game with positive expected value, individuals with significant upfront losses will lose over time if the reward percentage is too close to the loss percentage. Because your future wins depend on your present capital, which in turn depends on your past wins. This becomes an optimization problem!

This does not mean that you shouldn't play a game with positive expected value. Expected value is still the salient framework with which you should judge risk. It just means that the size of your bet needs to be considered in conjunction with your total capital, not just whether any individual bet is more likely to win than lose.

The author states this seems to not be well known in finance, but in point of fact this is very well known in both literature and practice. A trading strategy with positive expected value has additional considerations before you execute on it, including your total capital and liquidity.