The affix ṭhak comes after a word in the Instrumental -3rd case in construction, in the sense of 'refined thereby'.,
The word संस्कृतम् means 'refining or enhancing the quality of a thing'. Thus दध्ना संस्कृतं = दाधिकम् 'refined or made tasteful by curd'. So also शार्ङ्गवेरिकम्, मारीचिकम् ॥
The separation of this sutra from the last, is for sake of the next sutra, in which the anuvritti of संस्कृतं only runs,,