Actually that page suggests that they can't access it. They'd never passed the security on it if that page was lying and they don't encrypt it. Clearly there must be some kind of mechanism they can use to decrypt it for law enforcement. The technicals of that are what I was actually interested in from my original comment.
EDIT: Oh my God I just figured it out. It's not enabled by default. You have to explicitly turn it on per conversation. That's terrible
I thought messenger was end-to-end encrypted, at least according to Facebook. How were they able to hand over the chat logs? The messages should be encrypted with a key that is itself encrypted with user's password, which Facebook doesn't store.
What am I missing?