وَإِذَا عَطَفَ عَلَى الضَّمِيرِ الْمَرْفُوعِ الْمُتَّصِلِ، يَجِبُ تَأْكِيدُهُ بِالضَّمِيرِ الْمُنْفَصِلِ.
Translation: When coordinating onto an attached nominative pronoun, it must first be reinforced with a separate pronoun.
In ḍarabtu, the attached -tu is itself the subject. The text requires reinforcement before coordinating another overt subject onto it, absent an intervening separator.