minyan: I think he was responding to the idea that a ben nidda is supposed to be brazen. He was saying that the BTs he knows are not brazen, hence, there is reason to suppose that they are not a ben nidda.
I suppose the mother could have not been a nidda if she had gone swimming in the ocean. chatzizta b’miut is only drabanan, and we only require kavana lechatchila.