How does Salva know the woman in the orange scarf is not his mother?

From the text:

“Mother! Mother, please!"

Salva opened his mouth to call out again. But the words did not come. Instead, he closed his mouth, lowered his head, and turned away.

The woman in the orange headscarf was not his mother. He knew this for certain, even though she was still far away and he had not seen her face.

Uncle’s words came back to him: “. . . the village of Lou Ariik was attacked . . . burned. Few people survived . . . no one knows where they are now."

In the moment before calling out to the woman a second time, Salva realized what Uncle had truly meant—something Salva had known in his heart for a long time. His family was gone. They had been killed by bullets or bombs, starvation or sickness—it did not matter how.