As mingus88 said, this story is literally in response to Apple leaking messages sent through Signal. Doesn't matter if the message is securely transmitted if the operating system then keeps it lying around in plain text in a cache.

From the linked article:

> The independent news outlet reported that the FBI had been able to extract deleted Signal messages from someone’s iPhone using forensic tools, due to the fact that the content of the messages had been displayed in a notification and then stored inside a phone’s database — even after the messages were deleted inside Signal.