The letter is a means of notification requesting an extension. The Benn Act has been complied with and there is now a request for an extension until the 31st Jan 2020. Just because it was unsigned and a photocopy does not mean the request has not been sent. The not signing and sending a photocopy is Johnson's childish way of showing he does not want the extension. He has made that clear with his other letter.
https://www.bbc.co.uk/news/uk-50114538
"Regrettably, Parliament missed the opportunity to inject momentum into the ratification process for the new Withdrawal Agreement. The UK Parliament Representative will therefore submit the request mandated by the EU (Withdrawal) (No.2) Act 2019 later today.
It is, of course, for the European Council to decide when to consider the request and whether to grant it. In view of the unique circumstances, while I regret causing my fellow leaders to devote more of their time and energy to a question I had hoped we had resolved last week, I recognise that you may need to convene a European Council.
If it would be helpful to you, I would of course be happy to attend the start of any A50 Council so that I could answer properly any question on the position of HM Government and progress in the ratification process at that time.
Meanwhile, although I would have preferred a different result today, the Government will press ahead with ratification and introduce the necessary legislation early next week. I remain confident that we will complete that process by 31 October."