Something that is hypertextual has references which connect to other texts. It is best seen in the modern day through hyperlinks (or links) which can be clicked to quickly navigate to a page that is related to that which the user was originally on. It efficiently diffuses information across pages which are dedicated to conveying certain fragments of information, although all contribute to a greater whole.
A Zettelkasten must be hypertextual. It must be easy to crawl through, and its notes must have references to other concepts within the collection. This is easily performed in digital ZettelkΓ€sten via hyperlinks similarly (but different to) a wiki.