I agree with Andy about the issue of perhaps you were testing with a browser that was logged in. I don't think anyone could stumble upon a simple way to let everyone edit the site from the front end. If it does that's pretty amazing to me.
Apparently it just allows editing of rich text fields so far. The dev is still out there at scherpontwikkeling dot nl if you wanted to shoot him an email. Sounds like he was thinking about a system for full front end editing, very astute of him in 2013.
For this problem you could possibly consider hiding only the richtext field behind a password with the Personalize extra https://docs.modx.com/extras/revo/personalize,
that allows you to serve different contents to logged in and not users, you could have the rich text field for logged in users and a 'register now' banner otherwise.
Catching all the rich-text fields could be an issue though, I don't know your situation.
It could also be wise to use an extra with a community and support like newspublisher