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Well, the Glasses' weakness was arrogance. He thought he had Shioon all figured out and Shioon used that. That was the first time in the fight where Shioon used a feint to make it look like he was doing one thing but actually doing something else entirely. He made a motion like he was expecting Glasses from the left (the pattern Glasses used earlier in the fight and was trying deceive Shioon into thinking he was going to use again). That motion with the fist was a feint so that Glasses would think his foot tech succeeded in outwitting Shioon but Shioon saw through it this time and was ready to use the ghost step at the right time. It is not that he had time to react, it is just that he planned to use the foot tech ahead of time.
Glasses, due to his arrogance, didn't think Shioon was capable of more than he had already seen and that was his downfall. Once his leg gave out and Shioon got him with a full powered inner fist strike right in the vital spot it was over, at that point Glasses couldn't move and therefore couldn't avoid the follow up punch. Not sure what is so illogical about this sequence of events and that's coming from somebody who absolutely hates arse pulls and contradictions in anything I read.