Your phrasing seems to imply that

- we should know that messenger being "subject to community guidelines" means silent censorship

- Zuckerberg saying something publicly means it is justified

I would disagree with both of those implied assumptions

You should know it because you signed the terms of service which includes the statement, (in the beginning) that Messenger is subject to Facebook's community guidelines, which at the top of the page tell you:

"As people around the world confront this unprecedented public health emergency, we want to make sure that our Community Standards protect people from harmful content and new types of abuse related to COVID-19. We're working to remove content that has the potential to contribute to real-world harm, including through our policies prohibiting coordination of harm, sale of medical masks and related goods, hate speech, bullying and harassment and misinformation that contributes to the risk of imminent violence or physical harm. As the situation evolves, we continue to look at content on the platform, assess speech trends, and engage with experts, and will provide additional policy guidance when appropriate to keep the members of our community safe during this crisis."

https://www.facebook.com/communitystandards/

It's e2e but we can your messages before you send them. Original

Edit : it appears that it is not e2e by default

That messenger is subject to facebook's ~~community guidelines~~ world view ...