The idea is that athletes are rawer, more malleable, more flexible, but riskier and generally less skilled (therefore probably cheaper) employees when compared to true specialists. This makes them good hires for startups.
The article could have benefited from actually saying this.
But then the whole issue is confused by "athletes" being a a terrible term for "generalist" because while real athletes are certainly self-motivated, they tend to be extremely narrowly focused on a very specific role in a very specific activity (with a few exceptions that prove the rule), which doesn't mesh well with what the article is saying about hiring them as generalists for startups.
One of the decisions we made early on was to only hire athletes. Metaphorically speaking, that is. I don’t mean athletes as in people who play sports. Athletes, as in people that can play any position within your startup. It means someone is first and foremost a generalist. They are guys that think and act like founders because they’re not so locked into one individual role that they lose track of the wider needs of the company. They have lots of good skills that can be applicable all over the company but may not be the very best at any particular one skill.
So no a ninja would not necessarily be a generalist if we were to talk about ninjas...