It's like saying that you're not responsible for stealing a wallet because you didn't know exactly how much was in it.
The question of responsibility is of course not what we were talking about, but rather whether the statement "PFAS were known to cause issues" was in fact true. Too often we see possibility of harm being deliberately conflated with actually causing harm. Is it too much to ask for some honesty here?
Comparing them betrays a complete ignorance of the molecular properties that, in combination with biological system processes, make A and B6 healthy at small doses and poisonous at large doses on one hand, and those that makes PFAS linearly poisonous at any given doses on the other. (And yes, you can be exposed to small amounts of toxins and be fine. That doesn’t change the toxicity of a given substance.)
Struggling to see how this could be anything but a bad faith argument.