I think this may be the most important thing in the article. If the conspirators really were communicating via in-game ephemera, the communication probably wasn't encrypted at all. If so, the hurdle that the security forces faced is not that they couldn't access the communication, but that they couldn't interpret it.

And that just shows how little a mandatory backdoor policy would help: being able to read something doesn't mean you can understand it. It's always possible for someone to come up with a communication scheme that you didn't anticipate or can't interpret, and you can't legislate away that capability.

So a backdoor policy doesn't trade privacy for safety -- instead it trades privacy for a chance at safety, and pretty much just safety from careless, unprepared attackers.