So much of this discussion is simply going in the wrong direction -- The Afghan COAS is simply saying what is considered common knowledge and sense in Afghanistan - which should give you a clue how knowledge and sense are oxymoron when used in the Afghan context.
The more interesting question is Pakistan's role in fostering the kind of image and "knowledge and sense" it has fostered about itself among Afghans - Afghan society is rather simple, in so far, as what they think of Pakistan - they think it's a push over, that it's center is "mushy", "soft", "unauthentic" - now they only think this because Pakistan promote this impression --
Why would the COAS say the border is the Indus? Well, look at these anachronisms, these expediencies, these bargaining chips that are the FATA and PATA -- Are these really Pakistan?? Show me!! Pakistani state jealous and insistent that only Pakistani state authority will prevail there?? Pakistan government services available to all there??, Pakistani police and army "Willing" to assert themselves and their legal authority to assert themselves there??
See, friends, please take it a little easy on the Afghans - If you call them "brothers" then be brothers - help them and set them straight when it's required - and there is an ethic at work when setting them straight, if you want them to respect you, make sure they know you not only have the ability to inflict hurt, but the WILL to do so - they are quick learners, Firm but Fair, actually works.
Worried about Afghans with Pakistani ID cards? You should be - but you should be worried about Afghanistan failing to absorb the EXPECTATIONS of it's population and then you will see mass migration into Pakistan and Iran -- Can you blame them? Do Afghan fathers and mothers want less than a brighter future for their children?? And when, it's not IF, it's WHEN, they come, they will cause social disruption and political disruption and ethnic clashes, UNLESS, Pakistan's economy enables the absorption of these and government services, particularly education, socializes them in to the ethic of being Pakistani, not Muslims, but Pakistanis.