I just tried dragging a purchased book to Finder, and it only gives me a .webloc pointing to the book's store URL. On the other hand, if I manually locate the book in ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books, it appears to just be an (unzipped) ePub without any encryption.
It's up to the publisher whether content bought from the Apple store has DRM, not the store itself. Many books have it, but publishers like TOR and any books from Brandon Sanderson don't. There's sometimes a line at the bottom of the description that'll note it. For example, on Redshirts (https://books.apple.com/us/book/redshirts/id501758516)
> At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied.
Still, signing into public devices with your private credentials and data is a bad idea. What if they kick you out of the store or take away the device your were toying with before you manage to wipe/reset it?
The person you're replying to is suggesting "go to the apple store and use one of their demo computers to export the book", not "go to the apple store and buy a Mac".
https://discussions.apple.com/thread/7967098