Sure the Black Power Movement did not persuade people to support the movement, but that is exactly her point! She thinks that they have contributed in other ways, namely moving the overtin window to the right directions. If I have to make a comparison, the MAGA movement did not persuade people to be conservative. In fact, if you are moderate before MAGA, you might now turn liberal just to oppose them, but it might still be the case that MAGA has made other conservatives seem like more viable options for compromises. Like if you were Sanders or bust before, now you would be happy with Biden!