Considering there is a separate price for foriegners (RMB132 instead of RMB120) & an English registration page where you can enter your nationality & passport number but no place to enter your green card number, I don't think its right that this is only open to GC holders.
It seems you are correct! The system has been updated since we tried it yesterday to allow foreign passport holders - good news! The blog has been updated to reflect this.