Why does the final nūn in «لَيُنْبَذَنَّ» have ghunnah?

Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules

• It is an article lām
• It is followed by madd wāw
• That nūn bears shaddah
• It is followed by bāʾ