A proper name (ʿalam) identifies a particular person or entity, as طَلْحَةُ does. The book requires proper-name status for femininity expressed by tāʾ to prevent full declension.
A final ة by itself does not make every noun a diptote. مَدْرَسَةٌ is a common, fully declinable noun; طَلْحَةُ is a proper name.
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Causes of Diptote Status — Femininity and Its Conditions
A proper name (ʿalam) identifies a particular person or entity, as طَلْحَةُ does. The book requires proper-name status for femininity expressed by tāʾ to prevent full declension.
A final ة by itself does not make every noun a diptote. مَدْرَسَةٌ is a common, fully declinable noun; طَلْحَةُ is a proper name.