Why is طَمَاطِمُ treated as ghayr munsarif here?

Cause of Ghayr Munsarif: Sighat Muntaha al-Jumu'

• Because every food word is diptote.
• Because it begins with ط.
• Because its form matches صيغة منتهى الجموع even though it is treated as a singular lexical item.
• Because it is a verb.