@vanderwal so, I think I see where we went askew here.
You said, "Most services get this wrong and make the replies visible to B's followers only."
I disagreed, "Most services get this *right* and make the replies visible to A's followers only."
I don't think we disagree about the right way to do it -- we disagree if services actually do it that way.
I am not sure why you think they don't. As far as I can tell, X, Instagram and Facebook all make replies visible to A's followers.