> Similarly, note that filling in all three corners of that symbol is actually an equation. I'm also not aware of any other symbols that constitute entire equations on their own.

Multiplication and division can also be put into a triangle, such that filling in each corner constitutes an equation. See my comment [0] about the density equation. I suspect such triangles reflect some deeper relation of abstract algebra.

[0] https://news.ycombinator.com/reply?id=12138817&goto=threads%...