"All Lives Matter" was not in common circulation until "Black Lives Matter" entered the scene, because the phrase is used as a rebuttal to "Black Lives Matter." That's why it's commonly understood to be a dogwhistle. When person A says "Black Lives Matter" and person B says "All Lives Matter", person B is really saying "NO. All Lives Matter" the un-spoken "NO" is their disagreement with "Black Lives Matter." If they agreed that "Black Lives Matter" they would not have had to come up with a rebuttal to counter it.