JAIF

Either he evaded it, he partially evaded it, or it struck him, there are... no other possibilities, by any laws, excluding those that require him to... counterattack, or something - unless you are implying that he cut himself for some reason, Byakuya was hit by it, and if you are implying he took it head-on, the full force of the blast striking him, you are wrong, because the path that the Getsuga Tenshou carved into the ground is clearly visible next to him - proving it with a scan is an easy and painless process - which means, either he sidestepped it or the blast's course wasn't meant to hit him originally, which makes no difference - a "slow, unwieldy" attack meant to disrupt Byakuya's own not only managed to do so without any sign of difficulty, but also caused injury to Byakuya - you can say it took him by surprise, but I mean... you are a shinigami, you've been fighting for a thousand years, and you still haven't learnt to be prepared in case your opponent throws something like that? No, it's more like a) he didn't feel the neccessity to sidestep the attack, even after it pierced Senbonzakura's release, or b) he wasn't fast enough to avoid it completely - in the first case, Ichigo proved him wrong by injuring him, in the second, he is too slow to begin with, so your point is void.