The "that wasn't known for it" indicates that her career is generally a short-lasting one, instead of one where someone might expect to be employed for a very long time. "And she hadn't had longevity in her career by being stupid." This still conveys 2 of the parts of the author's sentence: that she has been in her career for a while and that she hasn't acted stupid. But it leaves out the information that her career is something that you wouldn't expect to last for a long time. Assuming it's a desirable career, the added meaning from the "that wasn't known for it" is that she is in someway impressive or better suited to that career than the average person in it, perhaps because most people burn out quickly or aren't good enough to last in that career. The writing is acceptable, but based on the other two sentences you posted, not particularly good or sophisticated.