In practical terms, he's talking about structure notes which consist of an outline of links to other existing notes.
Structure notes is a technical term. They're "meta-notes" in that they're notes that point to other notes containing content.
This seems to be what Ahrens means by a "sequence". I guess there's one explicit sequence defined by notes and their forward/back links, but structure notes can create used to arbitrary sequences.
This also seems to be the key mechanism by which a linear narrative is created.