By trader, I mean domain knowledge about the markets. Statistics is the toolbox that this domain expert uses to test their hypotheses and turn them into a profitable model. But if the person isn't a domain expert and only knows statistics, their ideas about what to test won't be good.

And to knowledge, I'd add disposition. It's been years since I've been in finance, but the best traders I worked with were all very driven to succeed, to dominate, to win. Markets were really interesting to me, but I never cared much about that part.

Yeah, it's a performance discipline like any other (competitive gaming, athletics, etc) where only the top few % can succeed. If someone isn't very driven then they won't make it.

This is almost precisely the thought process behind how my company hires data scientists who build user-facing analysis.

1/3 statistician 1/3 engineer 1/3 product person who can learn the user's domain-specific needs