I would actually say yes, it is a "gimmicky mix up". I would certainly insist that if you IB the attack before 6B, you should react to it and punch them in the face. I am, most commonly, more often rewarded for Jin throwing 6B than I am hurt; I would call that a weak point in the mix.
Arakune's 5A is 6 frames with a weak hit box and low reward. Also, Arakune's 5A mash would pretty much only beat Jin's 6B, where with many other characters if I IB mash I will beat almost anything, making the 6B weaker by proxy. But Arakune should backdash that shit easy, especially since Jin has very few answers to the backdash unless Ara is in the corner (In which case, he should spider super it).
Human tournament error is completely legitimate. You ever see a top player drop some easy combo and lose for it? Same shit, but on defense. Doesn't mean they didn't mess up.