This is DMs, most people don't think of those as private, and it's unreasonable to tell people to treat DMs as public. By the same logic email, SMS, voicemail and all "essentially public".
(Although I agree that the premise in a broader sense of trusting a Chinese company over an American one is flawed.)
Maybe still safer than a US company having the data, but not great.
No, it's not. People should treat unencrypted centralized service DMs as public.
TFA is the proof.
Any other belief regarding the privacy of them is flawed and incorrect.
And yes, it's well known that Zuck did abuse God-mode back in the day.