I think calling Factorio an idle game is really stretching the definition. There are some gray areas, of course, and no definition of videogames -- or anything

I think calling Factorio an idle game is really stretching the definition. There are some gray areas, of course, and no definition of videogames -- or anything, really -- has a clear-cut demarcation, but really... Factorio is an RTS/programming game. You could call it an automation/programming game disguised as an RTS. It requires thinking and adjusting, designing layouts, refactoring. It's really not what you think of when you think idle games. Really, that's the best way of thinking about it: if someone says "hey, recommend me an idle game, I've finished Cow Clicker and I'm ready for the next step", would you feel comfortable recommending Factorio?