i thought that was the case, at least my impression from the discussion about it was that once a string was marked secure, it would no longer be shared.
No, that is not at all how it works. They are still just as shared as normal strings.
So it is indeed not really all that useful.
In a multi user system like a MUD or similar you can probe for passwords (if they are marked as secure) without logging in by just testing if a string you create is marked as secure or not.