The site editor is not opt-in by default though.

For a brand new WP user, FSE, site editor and global styles are the default, and not using these tools is opt-out.

As most of the voices around this subject are seasoned WP users, I wonder what impression a new user would have when they see Appearance > Editor (beta)?
Is this different and separate from 'Open site editor' in the welcome panel? They look the same, yet one link is marked as beta. What's going on here? etc..