Though they initially seemed unnecessary, they were actually very helpful for thinking about how to parse through difficult stimuli/passages

@DorinLee Hi I don't know if this is helpful but I think by the statement "the only problem is that you made up your own premise" they mean that you alone had to assume that B-->A to make the conclusion valid. The stimulus only tells you that A--> B. The only way to make this conclusion valid is to add in this premise you made up (B--> A). So the conclusion is not valid given the premises the stimulus gives you.