Please read the articles. It already says that those are encrypted, as well as voice calls. Apparently once everyone is upgraded, nothing will be unencrypted. Their white paper also describes the protocols used in quite a lot of detail.
Maybe the next step is a more obvious indicator.
They do this to prevent degradation attacks.
Don't know what kind of funding they've been having, but for an open-source app, it's pretty polished.
So I fail to see the problem. It still likely protects you from nosy neighbors or nasty non tech savvy competitors, even if it doesn't protect you from state level actors or from Facebook itself.
The above, in addition to the fact that open source is neither required for auditing, nor guarantees proper auditing occurring (see OpenSSL having vulnerabilities for years before anybody released them to the public)