By that logic, if that culture you are referring to does not exist in Pakistan to any meaningful extent then Indian people can only claim it's legacy where it currently exists, else it died out and was superseded (Pakistan).
Indologists(both Indian and European) visiting Pakistan for archaeological and other data can explain it better. Sindhis and Punjabis of pakistan are Indic people but only ethnic relations except for Hindus of Pakistan. That's the only thing that connects us with Ancient India. But more recent History like medieval age and modern british era is our common heritage.
What else could jinnah or Iqbal call themselves, that's what the region was called.
the point was that the were In fact Indians for most part of their life and took part in Indian Independence movement NOT Pakistan Independence movement. nothing of that sorts exists for that matter. Pakistan the acronym only came after 1941. Objective History is good for everyone. For them and us. esp. our future generations.