Error in the CoreDataCloudKitDemo sample?

Code Block 
private func remove(duplicatedTags: [Tag], winner: Tag, performingContext: NSManagedObjectContext) {
    duplicatedTags.forEach { tag in
        defer { performingContext.delete(tag) }
        guard let posts = tag.posts else { return }
            
        for case let post as Post in posts {
            if let mutableTags: NSMutableSet = post.tags?.mutableCopy() as? NSMutableSet {
                if mutableTags.contains(tag) {
                    mutableTags.remove(tag)
                    mutableTags.add(winner)
                }
            }
        }
    }
}