Tarnished are definitively people who had grace, or their descendants.

This is something that doesn't even bear mention because all Tarnished originates from Godfrey & his warriors being divested of their grace and being exiled to outside the Lands Between.

Any Tarnished in the Lands Between are a product of what Varré describes.

Are you familiar with grace? The golden light that gives life to you Tarnished.

The exact same thing which is explicitly shown in the opening cinematic with Hoarah, Goldmask, Fia, Dung Eater, Gideon and us.

The grace which Marika stripped the Tarnished of having returned, resurrected and guided them back to the Lands Between.