Why is «عَبْدُ اللَّهِ» unlike the compound proper name in this lesson?

Al-Murakkabāt — Compound Numerals, the Twelve Exception and Declining Baalbek

• It is an iḍāfah: عَبْدُ is the first term and اللَّهِ the genitive second term
• It is the number twelve
• It is a past-tense verb
• It must be a diptote