Not Indigenous but as I understand it, in their cosmology, links between people and country (and between country and language) are permanent and part of the natural order of things. It's a logic of relationships rather than one of possession, and the permanent relationships were/are established in the Dreaming. You belong to country in a similar way to how you belong to a family. Same as I am always related to my family even if I'm totally estranged from them, they're always related to country.

The relationships between language and country were/are also established in the Dreaming. So Dharug Country is always Dharug Country even if nobody speaks Dharug on it any more.

At a more basic level, it's been theirs for at least 60,000 years. That's a lot of ancestors in the ground, literally becoming part of it.