Interesting! I would not have guessed that "novella" predated "novel". But it seems that the modern English meaning of "novella" being a story with a length in between a novel and a short story is pretty different from the older Latin and Italian meanings. I'm going to imagine there was another process where "novella" picked up that specific meaning.

It happens I watched The Decameron series on Netflix recently, which was entertaining. I'm sure it's very different from the original book - probably only sharing the setting, and some character names.