The affix aṇ comes in the sense of 'enounced by him' after the word kalāpi,
This debars णिनि of 4.3.104. Thus कलापाः 'who study the work enounced by Kalapi'.
The word कलापाः is thus formed:- कलापिन् + अण् ॥ Here comes in force Sutra 6.4.144 which declares that इन् of कलापिन् should be elided before the taddhita affix. This elision is, however, debarred by sutra 6.4.164 which declares that words ending in the affix इन् retain their form when अण् not denoting a progeny follows. Therefore कलापिन् would have retained its इन् but for this Vartika.
Vaart:- The टि portion of the following words is elided when a Taddhita follows:- सब्रह्मचारिन्, पीठसर्पिन्, कलापिन्, कुथुमिन्, तैतिलिन्, जाजलिन्, जाङ्गलिन्, लाङ्गलिन्, शिलालिन्, शिखण्डिन्, सूकरसद्मन्, सुपर्वण् ॥ Thus ईन् being elided, we have कलापाः ॥,