Its true that original abode of many pashtun tribes was sulieman mountains but we dont the time at which they expanded towards other areas, it could be thousands of years ago. But yea KPK was colonized by pashtuns in 9th/10th century, from eastren afghanistan. First dilazak pashtuns settled in northren KPK ,who were important allies of mehmud ghaznavi, and after four centuries yousafzais invaded the areas of dilazaks.
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The author is speculating. Bamiyan is in central Afghanistan, its buddist statues can not be used as example to prove that kandhar was part of gandhara. Kandhar's history is linked with persia. Gandhara was swat-peshawer-taxila, you wont find gandhara ruins beyond this region. There was kabul shahi hindu kingdom but hinduism itself didnt penetrated into afghanistan like buddism. That being said, i am no historian. My assessment might be wrong completely.