Facebook’s image CDN doesn’t work like this. If I save the media URL and send it to someone, it won’t work for them. I’ve tried — the URLs are user specific.
Sharing a URL to an asset is different than downloading that asset and sharing it directly.
But this article is about sharing the URL. That’s preventable, which screenshotting is not.
That's not been my experience playing with this on FB with photos restricted to friends-only viewing. Right-click, open image in new tab, copy that URL and you can view it from an incognito window that has never logged into FB on any account.