> Is there really anything particularly Japanese about these women's methods or is it just Orientalist branding?
I don't think these two are mutually exclusive, something can be authentically Japanese and be branded that way in particular to foreign audiences.
Kondo's "spark joy" method doesn't seem that Japanese though, if anything it seems almost made for Western audiences which isn't suprising given that she's three decades younger than Yamashita but the latter seems to genuinely have Buddhist influences, as she says herself in the article. It's probably also the reason Kondo is more popular.
But there really is a big emphasis on detachment, ephemerality and circularity in Japanese aesthetics, you can go back to writers like Tanizaki (In Praise of Shadows) and I think that comes through in the motivations that Yamashita gives for her method.
edit: and by the way the observation about Newton isn't unfair at all. English is too narrow, but philosophically Newton's deterministic and mechanical laws of motion are the geographical and philosophical product of modern European thinking. They would not have made sense in the world of Aristotle, which is why it took so long!