I think, in structural engineering terms, it refers to old natural stone wall [0] construction methods. Some bridges around me are made like this, and they're over a thousand years old.
But I agree that it's not a good adjective because I had the exact same first thought. Both "natural" and "stonewall" would be better, and they're not great names either.
I associate masonry with the old ages, since this is one of the technologies you research in Civ games and back then I think people did have stones in non-standardized shapes and were just trying to fit things together. I'm not native English speaker, so I first heard this term from the games.
There's no gravity here, so the comparison makes no sense. In walls masonry construction produces sheer lines horizontally, an axis gravity is not bearing upon. This feels like taking the metaphor just a tad too literally.
I didn't think so, of you look at old stone walls, there's an absence of continuous horizontal or vertical patterns. (However the discussion was a fork, about wall building, so it was quite literal)
"Dry stack" is another term for that, a stone wall without mortar.
In my opinion, thinking of this as a grid is misguided. It's barely different than flex columns. I would want to be able to have some objects take up more width than one column, or not have clean columns at all. Like "space filling" and "mosaic".
But I agree that it's not a good adjective because I had the exact same first thought. Both "natural" and "stonewall" would be better, and they're not great names either.
[0]https://duckduckgo.com/?q=natural%20stone%20walls&ko=-1&iax=...