The Pakistani identity belongs to the people and not any specific religion.
Even IF Pakistanis followed the exact same version of Hinduism as modern Indians, Pakistani ancestors are still unrelated to the Indian people, as is proved by the numerous separate kingdoms and people (with their own languages and cultures) who inhabited the Indus region through the centuries.
As for 'Hinduism', the religion differs depending on which part of India you go. It seems very unlikely that Pakistanis would be following any one of the Indian versions 1400 years ago. And this has been proved to be the case by RR several times.
Add to this the fact that a lot of Hindu Indians consider Jainism, Buddhism and Sikhism to be part of Hinduism, the definition of Hinduism becomes even vaguer.
The mounting evidence suggests that the Indus region has always had a separate identity right from the Indus Valley era. The region has continuously been inhabited to modern times by the same people and the region experienced countless of invasions due to being a crossroad of civilizations.
Certain members have a very limited understanding of the subcontinent which is portrayed by their dreams of colonial India being a natural country. As much as it pains us, the old kingdoms were neither tolerant or multicultural, hence the countless of wars. The idea of a democratic multicultural ancient India spanning from Afghanistan to Vietnam is absurd.
I found this interesting piece by dismayed Nepalese people:
Kathmandu: When Hindi comedy film 'Chandni Chowk to China' ruffled Nepali sentiments by wrongly asserting that the Buddha was born in India, it was not an isolated mistake, say many Nepalis. They feel that Bollywood has been stereotyping and belittling the Himalayan nation over the years.
Learn to appreciate our nation, Nepal tells Bollywood
Flintlock needs to understand that colonial India did not exist before British arrival. Painful reality, I know...