Opt-in encryption is no encryption.
https://vnhacker.blogspot.com/2016/05/security-and-privacy-i...
Of course that means it is just Google policy protecting the messages internally and it probably isn't resistant to a warrant, but the messages are protected from other access.
This is what stood out for me when watching the Google IO talk. It's about as encrypted as sending a plaintext email from one GMail user to another. Which is to say it's not "encrypted" in any meaningful interpretation of the term: Google has the plaintext, Google can change what systems access the plaintext and how; which includes complying with warrants - but also "becoming evil", if they find they have a business need to do so in the future.
It is nice that they at least provide some e2e encryption, but it's hardly much of a selling point of the service.
The idea is, our conversations that are not public, are private. Even if we're discussing some boring grocery list and not a top secret plan to take over the world.
That's a cute slogan, but I can't see any meaningful sense relevant to the present discussion in which it is true.