Right, but it's still by conquest, whether she believes it or not.
Cersei has a claim and so does Daenerys. Both are as legit as the other, they can claim birthrights and successions as much as they like but Robert took the throne by right of Conquest.
There is a reason that Robert went out of his way to murder every Targaryen he could find. Might makes right when it comes to conquest, but he always knew a Targaryen's claim would be seen as more legitimate and a threat.