The howto that you followed is incompatible with ISPConfig 2. ISPConfig 2 uses a linux system user setup to manage email accounts while you used a guide for apostfix virtual user setup. So your setup can not work with ispconfig 2 and recompiling postfix is not required for ISPConfig 2.
You wil have tp decide what you want? A virtual user setup, then follow the guide above and dont install ispconfig2. If you want to use ispconfig 2, then dont use the guide above and instaed ise a perfect setup guide for ispconfig 2.
If you want to use a virtual user setup for email with a controlpanel, then you might want to use ispconfig 3 and this guide here instead: