Rereading that answer and this interview with Spiner, it sounds like Spiner was just not one of the actors who were the suggested first choices by the creators/casting agency before they actually began auditions.
Brent Spiner, did however, go through what seems to have been a normal audition process.
Let’s go back to your TNG days. How quickly, or not, did you get a handle on Data and how to play him?
Spiner: I think fairly quickly because I realized pretty early on that there was really no precedent for it, that I could do pretty much anything I wanted and the audience would either accept it or they wouldn’t. But there was nobody to say, “Oh, that’s not what an android would do” or “That’s not how an android would behave,” because there weren’t any other androids at that point on weekly television. Even in the audition process, there was the question of, “Should we play this character like a machine, like a robot, or should we make him closer to being a person?” And we all agreed, finally, that it would be really tedious if I played him like a machine and a robot for seven years, if we went that long. So we decided to make him much more of a person who was growing in his humanity as time went on.
It sounds like despite the creator's initial choices, Spiner simply won the role with his audition.