وَيُكَبِّرُ مَعَ كُلِّ حَصَاةٍ.
Translation: And he pronounces takbir with each pebble.
The takbir accompanies each throw, rather than one utterance covering only the beginning or end. As the seven pebbles are thrown individually, Allahu akbar is said with each according to the described Sunnah procedure.
Kulli ḥaṣāh means 'every pebble'; one utterance should not be misconstrued as the seven individual takbirs described here.