Hi, thank you for posting this as it made me do some digging!
While it is listed in the API docs as an option (https://pub.dev/documentation/lemmy_api_client/latest/v3/PostListingType.html) and that's why it shows up here, it appears to have been removed from the main Lemmy codebase (https://github.com/LemmyNet/lemmy/pull/2377). So I think its inclusion is a bug. Will be removing in the next release.