- When you have a chat it has "Share" button
- When you click on the button it shows you a draft of the chat with "Post" button
- Clicking on the "Post" publishes the chat to public and sends you to "Discover" tab
- From published chat you can click on "send" icon to send link to the chat to someone else
IMO it is in fact dark pattern and goes against of how people perceive "Share" action. The fact you can't share without making chat public is also not cool.
For example top discover post I see right now is stylized picture of a baby, with original photo available if you open the post. I'm pretty sure the person who posted it was trying to share the picture with their relatives/friends.
Overall: Meta at its "best", better to say sorry rather than ask for permission...