We really need better journalism, this entire claim is based on another article where he says he has been talking to Moxie. They at least linked it, but you would expect a journalist to add his actual quote to the story. Here is the section from the linked article where they came up with the idea that he claimed signal was going to work with him.

  He went on to praise Signal, the encrypted chat app that is 
  run as a non-profit. He said he had spoken with its 
  creator, Moxie Marlinspike, who is now “potentially willing 
  to help out” with encrypting Twitter DMs.

  “Ironically, Moxie Marlinspike worked at Twitter and 
  actually wanted to do encrypted DMs several years ago, 
  [but] was denied that and then went and created Signal,” 
  Musk said. (For those curious, The Wall Street Journal has 
  a writeup from 2015 detailing Marlinspike’s time at 
  Twitter.)