What does it mean that atomic does not mean isolated?

Each atom exists in relationship to others — atomicity is about self-containment not loneliness.

Example: You have 200 atomic notes, each holding one idea. They sit in a flat list. None link to any other. When you search for 'decision-making under uncertainty,' one note surfaces — but you cannot see the five related notes on cognitive biases, risk tolerance, expected value, sunk cost reasoning, and reversibility of decisions that would turn that single retrieval into a usable argument. The notes are atomic, but they are also inert. Now add links between them. The same search surfaces one note, but you follow a connection to cognitive biases, then to sunk cost reasoning, then to reversibility — and in sixty seconds you have reassembled an argument that would have taken thirty minutes to reconstruct from memory. The atoms did not change. The connections made them alive.

Try this: Open your note system and pick any ten recent atomic notes. For each note, ask: what other note does this one support, contradict, extend, or depend on? Create at least one explicit link from each note to another. When you are done, you should have at least ten new connections that did not exist before. Now look at one of those notes through its links. Notice how the note means more in context than it did in isolation — the same content, but richer because of its neighbors.