I was also a little confused by the process here.

I don't know if this is a good reason to forgo the whole perfect experiment line of thinking or if it's just relevant to this one question, but my way of thinking of it was not in terms of experimental design but rather that A and D don't weaken the argument, they strengthen it. Either being true would strengthen the idea long-term treatment (or, treatment in general, for A) is unwarranted.