As I said mate, you are focusing on the differences, the author is focusing on the similarities. When he says cultural similarities he is not talking about the "burger people" of India and Pakistan, he is talking about the average folks. Keeping the whether Kashmir belongs to India or Pakistan argument aside there are still similarities between Punjabi and Sindhi communities of Pakistan and North Indian groups.
It doesn't have to be to exactly the same, its about who and what you can relate to. Would a Pakistani Punjabi feel closer to an Indian Punjabi, Jammuite, a Gujjar, a Rajput, a Haryanavi,a Pahari from Himachal or would he feel closer to an Arab or Persian? Does a Polish feel closer to a Ukranian or does he feel closer to an African?
Whether you like it or not, but there is a North India inside Eastern Pakistan and there is a Eastern Pakistan inside North India. Some see it, some don't or rather won't.
15 million Muhajirs-->Bihar, Uttar Pradesh, Deccan, Bhopal (300 Million Indians)
80 Million Pakistani Punjabis-->30 Million Indian Punjabis+2 Millions Jammuites+ Gujjars +Rajputs. Yes there are differences, but you can't ignore the similarities either.
35 Million Pakistani Sindhis--> 4 Million Indian Sindhis
1 Million Pakistani Gujaratis--> 60 Million Indian Gujaratis
Heck there are even some Baloch in India!
