If he signed an NDA, I probably wouldn't write a blog post about the interview process, that's a good way of getting fired for cause, since the late 1990s.
Here's the thing, posts like this are just making searchable what many people already know. The difference is that the tribal knowledge of how to get through interviews is one of the reasons why our profession has a diversity problem. If you don't know that you should read those books and plan for those questions, how well do you think you will do? Plenty of this wisdom travels over beers among the circles of friends in the industry, but putting it on the internet democratizes the knowledge for everyone.
We all know the interview process (across many companies) is a difficult / broken process. I think it gets even more difficult with scale the size of Google, Amazon, etc. How do you offer a consistent candidate experience and stable hiring bar without severely limiting the number of candidates processed or eating up too much developer time?
I'm sure we could find a dozen threads already discussing this on HN though.
This is primarily so their questions don't end up on LeetCode or other practice websites; they always end up there anyway, so it really just slows down that process and prevents them from having to think up new questions quite as often.