Why is the terminal wāw in آمَنُوا a madd letter?

Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf

• Sākin wāw follows fatḥah
• Wāw itself has fatḥah
• Only because a final alif is written
• Sākin wāw follows ḍammah