I read this as Notion and was like, “It’s fine, I guess, but breakthrough seems like a stretch”

Well I'm missing context to judge the impact. I wonder if there is something similar with code. Also looks like what it was, was a different UI to underlying equations.

Physicist here. The claim is not exaggerated: Feynman diagrams have changed the way quantum physics is done. Think of them as a layer of abstraction on top of mathematics. Each diagram can be translated to some lengthy integral, but those integrals are much too lengthy to be manipulated in all but the simplest cases. BTW, another, more accessible, case of notation being a major breakthrough is vector notation in geometry. Check out old textbooks (1930s or earlier) in mathematics or physics - you don't want to look at those equations if you grew up with vector notation.