I think GP means they stop at this point to ensure that they are giving all of the resumes a pretty fair shake by being fresh.
Afaik, any kind of slush-pile reading (including grading, which is probably the best researched) tends to get less fair as the process wears on the reader.
GP isn't optimizing for finishing the pile, but for making the most of what's in it.
I do. Credentials-wise I have BS in one of the STEM and currently enrolled in MS CS with intent to pursue PhD in Maths/CS. I have around 8 YoE and worked from Series B startups to IBs.
I want to give everyone a fair shake (including reading cover letters) and for me, resume fatigue sets in if I read more than 50.
If recruiting departments really suffer at parsing about 50 apps per day per recruiter, I can see why this got so bad so quickly.