In this example the idea that the Dalish are the rightful owners of Thedas is utterly laughable, ownership of land and the right to rule it are derived from power and the ability to muster support.
The discussion with Xil was over the elves taking umbrage over the discrimination they faced. Her line was in retort to how you said the humans being landowners mattered more, and that they shouldn't complain about their discrimination.
The Dalish can go around calling themselves the true heirs of Thedas, but it changes nothing.
You might be thinking of Genitivi, who wrote that humans were the "true masters of Thedas", which came across as racist when addressing a continent that houses elves, dwarves, and others.
The Byzantine Empire marched under the banner of SPQR centuries after Rome fell, and after them the Ottomans claimed to be the true heirs of Rome;s legacy, but since they couldn't enforce it nobody recognized their authority
Are we talking about the Orlesian control over the Dales? If they lose control over the Dales like they lost control over Ferelden, I don't see the problem.