"Did you lose an AirPod or your Charging Case? You can buy a replacement for a left or right AirPod or the Charging Case. Your replacement will be new."
"Replacement equipment that Apple provides as part of the repair or replacement service may contain new or previously used genuine Apple parts that have been tested and pass Apple functional requirements."
That's all I can find. Not sure if you get an in warranty replacement if it's new/refurbished.
It doesn't really matter because refurbished Apple devices are pretty much indistinguishable from new products. If you do get a refurbished item it'll have a new battery/case. Usually they only reuse the internals unless the other parts look brand new.