Timantic which is what they use to check at the airport states
Passport
Passport required.
Passport Exemptions:
Passengers with a Hong Kong (SAR China) Permanent Identity Card.
Passengers with an emergency or a temporary passport.
Document Validity:
Passports and other documents accepted for entry must be valid for a minimum of 1 month beyond the period of intended stay.
so you'll be fine, probably just bad hard coding in their online process