Do you need to use URLSession's dataTask(with: URL) inside of an Operation class's main() method?

Question

Do you need to use URLSession's dataTask(with: URL) inside of an Operation class's main() method? For example:

class Downloader: Operation {

    let postDetailsPage: PostDetailsPage

    init(_ postDetailsPage: PostDetailsPage) {
        self.postDetailsPage = postDetailsPage
    }

    override func main() {

        if isCancelled {
            return
        }

        let url = URL(string: "https://someurl.json")!
        NetworkingClient.shared.urlSession.dataTask(with: url) { (jsonData, response, error) in
            // work with jsondata
        }.resume()
    }
}

If the above operation is going to run on a background operationqueue anyway, isn't using dataTask(with:url) inside the main() method just overkill? In the Ray Wenderlich tutorial on Operations, they specify downloading data like so (see #5):

class ImageDownloader: Operation {
  //1
  let photoRecord: PhotoRecord

  //2
  init(_ photoRecord: PhotoRecord) {
    self.photoRecord = photoRecord
  }

  //3
  override func main() {
    //4
    if isCancelled {
      return
    }

    //5
    guard let imageData = try? Data(contentsOf: photoRecord.url) else { return }

    //6
    if isCancelled {
      return
    }

    //7
    if !imageData.isEmpty {
      photoRecord.image = UIImage(data:imageData)
      photoRecord.state = .downloaded
    } else {
      photoRecord.state = .failed
      photoRecord.image = UIImage(named: "Failed")
    }
  }
}

but in the apple documentation, it specifies never to use Data(contentsOf: url) for downloading data:

Is Data(contentsOf: url) a safe way to download data from within an Operation that will be run asynchronously on an operationqueue anyway (and for sure won't be called in a one-off block)?