There is more to unity than authorship, implied or causal. There is, for example, the logic or the abstract structure of the text. But for questioning these, Derrida and Joyce do quite well with their intertextuality, with textual games that depend on long text passages and linearity, and the fixed appearance of the physical page being there to be played off against.
It may be that so far hypertext has not done as well in this regard because so far there are not enough accepted conventions that can be broken and transgressed for effect. Writing hypertexts that play against the conventions of linear text will startle for a while, but only as long as hypertext is rare. We do not really notice anymore when movies violate the conventions of the stage; we are much more concerned with the relation among different film conventions. Perhaps hypertext needs to develop its own rigidities that can be resisted.
The medium doesn't do things automatically. Just having linkage available does not by itself do what that makes possible any more than having paper available automatically did what it made possible different than oral composition. Those possibilities have to be discovered, articulated, mutated.