This is a brilliant rhetorical pivot. Krishna says, 'Atha' (Now, if however...). He drops the high metaphysics of the eternal soul. He steps into the shoes of a pure materialist (Lokāyata or Chārvāka philosophy). He says, 'Even if you assume the self is just the body—constantly born (nitya-jātaṃ) and constantly dying (nityaṃ mṛtam)—your grief is STILL illogical, O mighty-armed Arjuna.'