They are based on the two UR pets we got last advent, so I believe its just referring to the fact that its based on UR pets, not that the items themselves are UR;
archive/2023/Special%20Releases/We did get
UR items last advent that are also cakes based on UR pets so its a bit understandably confusing, those are the only true 'UR' items that exist though. Whether a pet or item counts as UR is really just dependent on how its released. UR pets and items are released on Dec 18th during the annual re-release, and there is only a very small chance to adopt them, so any normal free item release whether everyone gets the same items isn't going to be considered a true UR.