It was brought up in the article. Elon only posted that after the security lead for the messaging product publicly tweeted completely wrong info about the status of the promised security audit.

Also, trust in the privacy of messages is the entire point of E2E encrypted DMs. It's a pretty dumb strategy to launch that feature and then walk it back with a "oops we're beta testing, don't trust that while privacy thing yet." So either Twitter/Elon has a dumb strategy, or they're (still) making wildly silly blunders as a result of self-inflicted wounds. Either way, pretty compelling arguments to never trust this feature even once they say you can.