One idea per page is a prompt to create modular notes that can be combined and recombined. Not necessarily brief but concise and internally coherent. The issue is that notes also need to synthesize multiple ideas.
One idea is context dependent which makes the use of the single idea page
But we also have paragraphs within notes, so an act of synthsis is possible n the page.
One idea per page is probably better stated as "try to make the note cohere so that it might better to link to other notes." And "this doesn't need to happen at the first pass."
So modular notes aren't dependent on the internal structure of one idea so much as being capable of working in multiple configurations. That's a rhetorical problem of How to Write Modular Notes.
Counterpose this with the Topical Notes or the old writing by topics. The topic might become not a note but a cluster of notes.
The whole matter is how to write at the next level up from the note - forming them into topics, arguments, exposition.
Writing with Modular Notes might be a way of thinking. Or we might just leave that as another matter.