I believe this divide is much more older than 1600 and goes all the way back to the Indus Valley Civilization.
Even if you look in the Mahabharata, there are many examples, one being of how it speaks about how the three main kingdoms of the Indus and some Central Asian tribes share the same cultural affinity.
"The Prasthalas, the
Madras, the
Gandharas, the
Arattas, those called
Khasas, the Vasatis, the Sindhus and the
Sauviras are almost as blamable in their practices." (8:44)
To a man with a hammer, every problem in the world looks like a nail. If you are really serious and are not twitching out factoids to make an all too specious a case, you will be able to discern in these descriptions the gradually increasing space, cultural, religious and linguistic, between the tribes who had stayed behind, the tribes who migrated but stayed in touch and the tribes who also migrated but were gradually, steadily gravitating to the increasing attraction of the powerful, iron-wielding eastern lands. The Mahabharata still describes an upper Indian political scenario where the great power centres were roughly centred about Delhi, the older regions to the north-east, the west and the south-west were known, and seen to be drifting away, and the power of the eastern states, that of the hero Krishna's maternal uncle, Kamsa, being an example, the gift of the powerful kingdom of Anga to the hero Karna being another, was slowly increasing. This was a prelude to the growth of the menacing power of the Magadhan state, that in time crushed all others and built the first not-from-epic-sources Indian empire, perhaps some nine hundred years later.
The relationship between the events - if ever they existed - and individuals of the Mahabharata and the great period of Sandracottos/Chandragupta and the rise and rise of the Magadhan state might be compared to the relationship between the probably Mycenaean heroes of the Iliad and the Odyssey of Homer, forebears of the Greeks of the Battle of Marathon, or the Battle of Salamis, or the glorious and lesser-known Battle of Plataea.
To return to your point, what you found described was not the inherent and permanent difference between two racial stocks but the slow drifting apart of friends and neighbours increasingly separated by time and space. The Gandhara that you cite gave the imperial family a daughter, Gandhari, and an astute but crooked manipulative brother-in-law to the High King, the uncle of the Kauravas, Shakuni. They were not different stock, just far-distant kinfolk living too far away to keep in touch with the politics of the centre.
Later, during the battle proper, we are introduced to the ferocious horse-borne warriors, the Parama Kamboja, hard-riding cavalry that could be staved off only with great difficulty, only by the united efforts of several of the heroes. Already these kinfolk were drifting away for each generation spent away; by 600 BC, Panini the grammarian, reputedly from those same parts, was able to discuss verbs and verb-forms that were 'archaic', and no longer belonged to the rigidly-defined derivative of the Indo-Aryan in which the Vedas were composed, a derivative which came to be known as 'the polished tongue', Sanskrit.
Again, it needs to be emphasised: what has been singled out and described is the natural linguistic phenomenon of formation and differentiation of dialects of the same language. In much the same way, Indo-Iranian had split into Iranian and Indo-Aryan, so close even within the Avesta and the Rg Veda forms of each that scholars of the one could follow the other, indeed, so close that knowledge of the Avestan form of Iranian was helpful for a full understanding of the Vedas, and could guide one through the rapids and shoals of Vedic Indo-Aryan, and precisely the same situation holding for knowledge of Indo-Aryan, the matrix from which Sanskrit was derived.
It is the same situation, with regard to social custom, ritual practice and with regard to use of language, that may be seen in real life in the differences in dialect between the one that is native to me, the dialect presently spoken in Dhaka, and the over-cultivated one spoken by the gentry of Calcutta.
These differences are not good enough to serve as a dividing mark between neighbouring ethnicities.