The latest generation, Kindle Paperwhite, Kindle Scribe 2, Kindle Scribe 1, and Kindle and the upcoming Kindle Colorsoft, all have something in common. When you plug them into USB into your PC or MAC, they no longer appear as external drives. This prevents users from using file managers to back up their books or to sideload new books onto the Kindle. Amazon has also removed the download and transfer via USB option for purchased ebooks from the content page. This will likely prevent people from stripping the DRM from the books and sharing them on piracy websites. This is not
Article says you cannot side load books on Apple Books. That is incorrect. You just send an epub to books via the share menu on Mac or iOS and it loads it. Also syncs it via iCloud if you want it to.
Perhaps the author meant you cannot download purchased books off of Apple Books.
Article is wrong about many things, seems poorly researched or the topic not fully understood by the author