A little bit of a clickbait title, the alternative title “The World’s Greatest Filing Cabinet” is more truthful, they invented a precursor of Wikipedia, kind of: How Belgium Nearly Invented The Internet - YouTube
Interesting - the encyclopedia by that time was already a well-established and popular form, so something here is being added - is it the categorisation, or the linkages, or perhaps the scale? Perhaps it’s the potential dynamism - a card index can be updated readily, compared to a book.
Found this image by Otlet, along with many others:
I think it is the dynamism. Also, I think encyclopaedias were more for “worthy” subjects, big lengthy essays written by experts about the Romans or the history of mathematics, that kind of thing, while this effort sounds more comprehensive.