I have to say, the article itself was so abstract I hardly found it worth reading. I did, however, check out the book mentioned in this article. A nice paragraph from the foreword sort of defines hypermedia:

"The World Wide Web is driven by hypermedia: the ability of a document to describe its possible states, and its relationship to other documents. Hypermedia is not just a way of making websites that average people can use; it’s a new style for distributed computing, powerful and flexible."