The thing is, ISBNs map to:
- publisher
- assigned title
- (roughly) order of publication
That's all that they communicate --- there is no hierarchy here to aid in discovery or to organize the content (and further complicating things, the same text may appear multiple times in a different binding --- a differentiation which is immaterial to an e-book).
The elephant in the room of course is the matter that "Anna's Archive" is not a legitimate book repository, but a piracy site, so what they are showcasing is how compleat (and brazen) their theft (and attendant lack of compensation) is.
This would be far more interesting if it were based on an hierarchical system such as LoC, and instead afforded an interface for accessing legitimately available books as are available from https://www.gutenberg.org/ or listed at: http://onlinebooks.library.upenn.edu/ or worked on at: https://www.wikibooks.org/