This is a fallacy based on the lie that “it was all once India”.
Pakistan and India have never been one country, they have simply been part of the same Empire under mostly foreign rule.
A Country and an Empire are two completely different things.
My region of Pakistan was once part of the Greek Empire does that make me the same as Greeks?
The British Empire in India and the presetnday Republic of India are two completely different entities.
If anything Pakistan existed before India because we got our independence on 14/08/47 and India on 15/08/47.
Even today India and Pakistan are not nation-states in the truest sense, there are a multitude of different ethnicities in both countries so they should both be multi-national confederations.
There is no one cultural/ethnic identity which binds the whole of India or the whole of Pakistan, let alone Pakistan and India.
We only have any real culturo-ethnic similarities with the small part of India directly bordering us which is occupied Khalistan (Punjab) and the Urdu-Hindi areas.
Other than Punjabi and Urdu-Hindi we have no similarities with India. A Pashtun has nothing to do with a Tamil and vice versa.
Pakistanis mainly mislead into believing we are the same because of Hindi movies and dramas but we understand these better than Indians, a lot of Indians (such as my Keralite friends) can’t even understand Urdu-Hindi.