Because the piece isn't very well written†, it's unclear whether the "infinite loop" of mediocrity she's referring to is something she actually observed, or something she surmises is possible. She is clear and specific, presenting numbers, when talking about things she was personally involved in.

Since the cycle of increasing mediocrity has a prominent float in the parade of horribles conjured by the "anti-diversity" (for lack of any better term) side of this debate, I'm left assuming she didn't see that occur. But she could also clear that up easily.

Finally, an obvious point: evaluation of the performance of an individual software developer is one of the great unsolved problems of software engineering. Virtually all performance evaluation done today is at root subjective. Subjective performance evaluations are easily tainted by prejudice; in fact, you have to work hard not to taint them.

If you think that's different at Google, re-evaluate: Google also runs one of the most famously capricious hiring programs in the industry. Despite constant rituals and genuflection towards data-driven decision making, Google continues to thrive based on its status as a premiere destination for new software developers, despite running a hiring process renowned for the quality of the people it has alienated. There is ample evidence of Google having scaled broken processes.

† 99% of what I write isn't well-written either, in case this sounds like a jab at the author, who I am not familiar with.