Easy, most of the code we write is pretty mindless. Boilerplate tying together of various internal libraries to make it play nice that you've done too many times to count. Following someone else's wiki page instructions on how to implement some "best practice" that needs to be done for a production system. Writing CRUD operations and such. Writing unit tests for all these things. Sure mistakes happen when you're exhausted but the stuff is hard from mentally demanding given the bar that they're supposedly hiring for.

I don't say that disparagingly, this is 90% of my day to day life as a mid-level engineer at a FAANG. The other 10% I might get to do something "neat" that I can justify somehow. It depends on the team of course, but I find the Author's experience believable.