I think that Amir realizes a couple of things about the Taliban official at the soccer stadium. The first is the description of the Taliban official wearing "John Lennon sunglasses." This brings to light how a note of hypocrisy permeates through the Taliban rule in Afghanistan. Rooted in the supposed indigenous and anti- Western position of fundamentalism, the noting of the eyewear as more Western than anything else helps to illuminate the rule of the Taliban in Afghanistan. At the same time, Amir recognizes how violence has become embedded in the Afghan life under the Taliban. There is an insinuation that his return to Afghanistan, similar to the moment where his emotional departure was evident, will involve violence. He sees this in the stoning of the adulterers, led by the Taliban official. Life in Afghanistan, like the life around his father's home, has become nothing more of "fallen splendor."
Amir's personal realization regarding the Taliban official will come in the next chapter. When he is able to speak with the Taliban official, he recognizes him as Assef. The town bully has become a leading member of the Taliban, and this realization is something that Assef fully grasps in chapter 22.