I think idempotent wasn't a good word but I think the idea being described is that crafting is acyclic.

For example Copper Wire + Iron = Green Circuit.

There's no recipe chain that takes in Green Circuits and produces either Iron or Copper Wire.

So once produced, you need to find a higher function for your products. You can't 'unmake' through a cyclic recipe.

Contrast that to ONI's water cycle, where clean water gets made dirty then cleaned again.

So ultimately in Factorio everything is leading toward the only resources that get destroyed - Science, Rockets, or Coal (or other energy types).