It's missing the point of the story to focus on this aspect. The characters involved in this event were not using Signal because they thought it was secure. They used Signal because they intended to break and knew they were breaking the law.
What you say is true. But if a technique makes it so that 1) they don't preserve a record for the future, and 2) they do leak (or risk leaking) information that can kill service people, I personally care more about #2.
(Ironic that, in trying to not leak to future investigators/prosecutors, they increased the risk of leaking to foreign adversaries. Shows which threat they're focused on.)
