He was brilliant, probably slightly autistic, spoke multiple (human) languages fluently (that he had taught himself, not grown up speaking or learned in school), and easily wrote 500-1000 lines of great code per day (not that that's the metric, but that's what I'd see when running `git pull`), implementing feature requirements in a front-end web application. His work truly had a "wow" factor. Give him a very big job that would take the average 4-person team more than 6 months, and it would be done in less than a month.
As a senior developer on that team, I quickly learned that the best use of my time was to remove roadblocks and streamline his workflow.
I worked across from him in an office for a couple of weeks one time, and was impressed at how focused and disciplined he was -- more than anyone else I ever saw. Every moment of the work day he was scrolling and typing into his editor. A quick break every few hours, and then back to work.
A 10x developer doesn't (usually) write 10x as much code. They get 10x the work done in 1x of the time. The quantity of work output is famously not measured in SLOC.
I've had the pleasure to work with a few. We all know the names of most of the dozen of 25x or 50x developers that exist in the world.
I think this person is writing to their preordained conclusion that the idea of a 10x developer is a myth. It's not.