Share buttons offer no inherent privacy settings.

Sharing to a text message is private. In contrast, sharing to social media platforms such as Twitter, Reddit, Pinterest, and LinkedIn makes the content public. The destination determines the audience.

The TYPICAL behaviour when you hit "share" on any platform is not to immediately share. It TYPICALLY gives you options to share to a variety of other sources, both public and private. It also generates a link if you want to grab the link and specifically share that.

That is the TYPICAL share behaviour. If what META is doing with their new app is obscuring this typical behaviour and a "share" click directly going to the public, that would violate the defacto behaviour users are accustomed to when using the share button.

The initial "Share" click doesn't post anything publicly.

It just opens a modal so you can choose to post. You have to make a second click to confirm.

Typical behavior on a Meta/social app is that it shares it to everyone. See Instagram, Facebook, TikTok, Twitter, etc.

If you index on chat apps, you're correct, if you start from Meta's social apps, which they said they have, you are incorrect.