The affix denoting 'enounced by him' is elided by luk-substitution after the words kaṭha and caraka,
The word कठ would have taken णिनि as it is a वैशम्पायनान्तेवासी word: and चरक would have taken the affix अण् ॥ Both are elided. Thus कठाः 'who study the Chhandas enounced by Katha'. चरकाः &c.
The word 'Chhandas' qualifies this sutra. Observe काठाः चारकाः श्लोकाः ॥,