Yes it is.
A “public post” is when the message was directed towards anyone who cares to listen.
A “private post” is when the message was directed to a specific group of recipients. The length of the list of recipients doesn’t grant a non-recipient access rights to the message.
This is easy stuff.
When you post something to "your" feed, you're sending it to Facebook with metadata that says "please post this on my wall" or whatever.
To strain an analog analogy, this is not like the telephone or even the post office, where you hand them something or send out voice packets and they just look at the recipient and forward it on to the actual person. Everything you send is sent to Facebook and kept at Facebook.
Replace "Facebook" above with any social media platform or cloud service. They're fundamentally the same. I don't consider iCloud photos private. You're sending your content to Apple, not putting it in some safe that you alone control.
An interesting weird angle: If X deletes their whatsApp account messages fail silently. FB stores the message under the false pretense that they are able to forward it to X. I think imessage does the same?
More important I think is the kind of "control" that's exerted: it of course should be around developing factually accurate sources and promoting those instead (maybe a warning "This post contains keywords detected anonymously that suggest misinformation. Here is an alternative position"
The fundamental problem is that we are building our entire lives around a few systems that are completely opaque. Facebook, Google, and many other algorithms are still closed source, and to their own detriment they cannot reveal details of their inner workings. That's why we need a way to move to a radically open society that still allows for innovation and advanced technology.
I also think legally, but this may vary from jurisdiction to jurisdiction, a message to one thousand people that you do not know personally and you are not in an organisation with, or that organisation has an open access policy (even if there's an entrance fee) would be a public message.
Of course, IANAL and just learned this from reading the news.
More interestingly however, is what Facebook or other media should consider public. In my opinion a WhatsApp group with 1k members should not be considered public, even if there's absolutely no bar to entry. Private companies should have no business policing private communities. If they've got concerns they should invite law enforcement to decide if there's any laws that are being broken.
The moment when communication becomes public is when the communication goes outside the boundaries of the group. If my antivax aunt posts an edutainment video about how vaccinations cause autism and it is clear that it is misinformation, and that edutainment video is not just in her group of weirdo's, but actually shared on her public timeline, in my opinion Facebook should definitely come down with the ban hammer. There should be a little "report" flag that I'll use to snitch on my aunt. Even if it's on her "friends & family" timeline I wouldn't consider it private for the purpose of culling misinformation. She should specifically select a group of people who have self-selected to be in that group for it to be considered a private message.
Also, if a private group on a platform like Facebook/Whatsapp has members that are underage, and not all members are in a complete graph of direct friends/family, Facebook should require that group to have active moderation that conforms to some sort of platform wide code of conduct.