If the high earners are the high risk takers, the study should also consider the high risk takers who went broke and therefore are not traders anymore. Still, it is interesting that behaviors differ so much.
The study, by itself, doesn't tell us whether hiring low digit ratio traders would be more profitable for a firm, or even if the average lifetime earnings of someone entering the trading profession would be higher for someone with a low digit ratio.
No, it doesn't. The factor it cites is testosterone sensitivity:
Previous research has found that the digit ratio reflects how much testosterone an unborn baby was exposed to in the womb. Those exposed to high levels of the hormone are more sensitive as adults to testosterone