Ok what if I had said "soap is literally just a fat (e.g. deer fat) combined with a plant that, when placed in water, will kill fish."

It's almost equivalent but told through a different cosmology/epistemology

I think a better example would be bread. Nearly every culture observed has independently created something like bread, which is just flour and water that is kneaded and baked, and if they don't have bread they probably have noodles, which is just flour and water that is kneaded, stretched and cut into strips.

Nah it would be pretty easy to discover soap accidentally. People have been using ash to scour metal for ages. Someone scours a plate or bowl with some leftover fat or oil or grease in it and it cleans really well, because it makes a kind of soap. It only takes one person to notice this fact, and the idea catches on.