Just like 'ninja' is a marketing word. It probably means the person loves their job and makes it their passion and others think that person is so good at what he/she does that they admire him/her. I wouldn't read too much into it. The wording is often chosen so the job ad sounds young, cool and hip (and the same founders might present themselves in suits in all public photos to appear more serious).

You're right, in reality the companies are looking for humble 10x programmers who get their job done without much fuss. Bonus points if the person is cool to hang out with outside work.