whereas I don't think I have ever seen "poltroon" used in any fiction that was set more recently than... maybe possibly the 1800's, but in my head it's way more associated with Middle Ages - ish historical fiction.

I guess Narnia is Middle Ages, but learning the word so young did nothing for my sense that this wasn't modern English. :P

I knew the device of course, but not the name for it.

Wikipedia informs me there are like 5 names for it, but praeteritio is the one I learned.