During setup of the add-on I selected to only sync unredeemed keys, which worked fine as far as I can tell. However, after going through my Humble account and redeeming, as well as gifting, a lot of keys, I expected the add-on to remove these keys after a library sync, which is not the case.
After a library sync all of my keys redeemed since the initial sync/installation of the add-on still pollute my Playnite library.