How to center text in xfce desktop?

Based on the quote you provided, it seems that the setting /desktop-icons/center-text should not be in xfdesktop-settings. Instead, it must be accessed via xfconf-query.

Strangely, it seems that the property /desktop-icons/center-text must be set to false in order to work properly with Flatabulous. This property does not seem to be created by default. In order to make it and set it to false, you can do the following:

xfconf-query -c xfce4-desktop -p /desktop-icons/center-text -n -t bool -s false

You should see the change immediately, but you can log off or reboot if it is not showing up.