There was a lot of travel from India up to the Silk Road and this route was the primary route for travel between China and India. Buddhism came very early to Afghanistan.
My point was this and only this: if that monastery is 2600 years old, and was always Buddhist from its beginning, then Buddhism came to Afghanistan before it even began -- because Buddhism itself is 100 years younger that the monastery.
It's only a minor point, I know, and I meant it as such.