An isolated note is a fact you have to remember exists in order to find it. A linked note is reachable from everything near it in meaning. The value of a knowledge base is not the number of notes; it is the number of useful connections between them.
Why it holds
Retrieval is the whole point of an external memory, and you retrieve by association, not by recalling a filename. A note linked from three related notes surfaces whenever you work on any of them. A note linked from nothing surfaces only if you already remember it, which defeats the reason for writing it down. So the link, with a sentence saying why the two ideas connect, carries more of the long-term value than the note body does.
Where it breaks
Links have a cost. A note linked to everything is as useless as one linked to nothing, because the graph around it says nothing specific. The goal is the few edges that carry real meaning, each justified in the surrounding sentence, not maximum connectivity.