You’re close. What it actually does is retrieve the father’s primary surname, if available, and return that through the name variable. You can check this on GitHub, where you can click on every procedure, to see what it does. And in this case, that means that you need to click on preset_name.
When you do that, you will see that preset_name goes for the surname, so it’s not good for your purpose.
Note that this code is only invoked when you add a child in the family editor window. When you add one from the relationship view, like I mostly do, another path is taken, which will always use the father’s surname, regardless of what you set in preferences.
This bug was registered and confirmed two years ago: