The book says الْأَلِفَ قَائِمٌ مَقَامَ السَّبَبَيْنِ: feminine alif has the force of two causes here—femininity and the permanence of that feminine marker.
Thus حُبْلَى and حَمْرَاءُ do not need an additional proper-name cause to be diptotes. The rule concerns feminine alif, not every word with some kind of alif.
Opening interactive controls...
Causes of Diptote Status — Femininity and Its Conditions
The book says الْأَلِفَ قَائِمٌ مَقَامَ السَّبَبَيْنِ: feminine alif has the force of two causes here—femininity and the permanence of that feminine marker.
Thus حُبْلَى and حَمْرَاءُ do not need an additional proper-name cause to be diptotes. The rule concerns feminine alif, not every word with some kind of alif.