If one reads that statement carefully, that says nothing about whether Facebook can read it before encryption. It only says they wouldn't be able to decrypt, once encrypted.
Read a bit more carefully and you'll see "decrypt" isn't in quotes, and as such is my word, not Zuckerberg's.
The keynote's online. (https://www.facebook.com/FacebookforDevelopers/videos/422572...) He mentions end-to-end encryption a variety of times, but one example is at about 15:23, where he states, and here I do quote, "without having to worry about hackers, governments, or even us being able to see what you're saying".
Now, skepticism about Zuckerberg and Facebook is warranted, but my recollection of the keynote is that statements like this didn't leave much wiggle room on this particular point. They were playing word games in other areas, like abusing the term "interoperability" to mean "between the different Facebook-owned apps", but I don't think they were here.
