Then more applicable advice seems like it could've been "Everyone should go to their Kindle library and download their ebooks as soon as they purchase them."
I don't mean downloading to Kindle, but actually downloading the book file to your machine where it is no longer at Amazon's mercy. If the book has DRM Calibre takes care of that, it's how many people transfer Amazon bought books to other ereaders.
It is a setting the author chooses when publishing their book through KDP, and some authors choose not to enable it. Pretty sure it is enabled by default though.
IIRC they can remove downloaded books. Better advice is "no non-free software in the house" and "mind your external dependencies" but that's a bit too extreme for most people.
https://theguardian.com/technology/2009/jul/17/amazon-kindle...