They don't need to backdoor it. The re-transmission with a different key feature is the very definition of a vulnerability. For as long any encryption protocol allows for it to happen, one can trigger a key renegotiation that can be taped.

And it was used few times already. Some of such occurrences were well documented.

In 2015, in a lobby discussion on DEFCON two people confronted Marlinspike about the retransmission vulnerability in the Signal. He was asked to give a _yes or no answer_ to whether the central server can trigger the key renegotiation by sending the "I lost my phone" command to both parties. And he answered this question _no_. This was long before the Guardian lashed at FB with the backdoor article.

I personally verified this account with 2 people.

Marlinspike Moxie is a liar.