> I have never encountered a single French person who uses anything but their first name as their "Prénom usuel".

Most people who chooses to not use their first name as their "prénom usuel" wouldn't disclose to people their aren't close with that they don't use their first name.

You wouldn't know unless they told you their full name. Witch, let's be honest, rarely happens outside of administrative work.

It's totally possible that you encountered multiple peoples using a "prénom usuel" witch is not their first name without knowing it wasn't their first name.