The sodium hydroxide left in there would probably turn any remaining fats into water soluable soap (and biodiesel), but it’ll also destroy any copper pipe it spends a lot of time in contact with as I recall.
That’s interesting, but would you ever expect to find drain pipes in copper? My house came with a box of old scrap parts, one of which was a thick metal trap for a sink. It wasn’t copper though. All drain pipes I’ve seen in the house are PVC (mostly gray, some white and some orange).