There is a lot of fetishization of this terminology in the spaced repetition community. It's really best to ignore it, it's not based in much.

I think their mental metaphor is that cards allow you to memorize nodes, and understanding is having a feel for the entire graph. But cards also help you to memorize links between nodes, subgraphs, overviews, principles, etc...

I also think it's mostly a ready made array of excuses to read off to somebody who is having a crisis of faith about whether a Anki is helping them or not: you're holding it wrong. You haven't put in the work. Are you making your own decks, you can't use other people decks because making your own decks = understanding (for mysterious reasons, do you really understand something you can't remember?) Are your facts atomic enough? Basically direct or indirect paraphrases of the Supermemo wiki.

Supermemo didn't discover anything, he computerized something that desperately needed to be computerized, and at that point wasn't restricted to the algorithms that could be executed by shuffling around physical cards, such as Leitner boxes (which are awesome, still, by the way.) His analysis is great to read and often insightful, but is no more profound than many others and often far less scientifically grounded. People just are addicted to parasocial relationships with self-improvement gurus.