Malbim: She did not understand the matter of Yibum, only that he is a relative, therefore he does Chesed with them to sustain them.

Toras Moshe (Bamidbar, Rus DH Gam Ki): She said so in astonishment. You say that he is Karov Lanu and mi'Go'alenu. I am Mo'aviyah, and forbidden to Yisraelim. Did he tell me to cling to his male youths? No, rather, to the females! Na'ami answered "Kalasah" (22) - you are my daughter-in-law b'Heter ¹ . Bo'az said Na'arosai due to modesty (to avoid mixing genders).

Malbim: She was from Mo'av; there, girls do not distance from boys. She did not infer precisely from his words. She thought that he intended with his people; the males were primary to him. She thought that one of them will marry her. This is why she is called ha'Mo'aviyah; a Bas Yisrael would have understood, both Na'ami's words "Im ha'Mesim" and Bo'az' words "Im Na'arosai." In any case, since he told her that she will collect until the end of the harvest, she proved like her mother-in-law, that he will do so because he is their relative.