Verse 19

19. This unborn (Advaita) indeed becomes modified or, different through Maya, not otherwise under any circumstances. If indeed it were to be modified in reality, the immortal would go the way of mortality!

That the primal Singularity of Existence is Reality, it is the ultimate truth, is being restated here. This primal Singularity has the logic of being unborn, without, origination therefore Eternal. It is accepted that due to Maya there is the appearance of duality.
But it is being said that if the reality were the other way, and duality was the normal state then immortality could not exist because to be immortal, eternal, one has to be unchanging, immutable, because that that changes has to have origination, a beginning. By accepting that eternity exists one has to accept that the primal entity is without origination logic dictates that this will be a singularity. There would not be two "existences". Therefore duality is the delusion or Maya not the other way round, singularity or Advaita as .