They refered to all the people living on that side of the Sindhu as Hindus (a geographic parameter has to be given). The ancestors of that region would be ours, since their literature, language and culture has been passed to us. Indra is a god worshipped in India, Agni and Prithvi are names of missiles, besides being worshipped in India in the same way they were 3000 years ago (by a purohit who chants mantras from the Vedas).
Anyway, today’s political map of Pakistan is not an indication of any “claim” or disclaim. Just as Pakistan’s national tongue, Urdu, comes from India, India’s mother of all languages i.e. Sanskrit, its sacred text i.e. RigVeda, comes from the territory which is today called the political area of Pakistan.
I guess like if the region of Kurukshetra was made to fall in modern Pakistan by the British official (Radcliffe I think) who was drawing boundaries just before partition, you may have said that Mahabharata is not a Hindu epic, but a Pakistani one !
Persians/Mughals did not confer the term Hindu merely to those people who lived on the banks of the Sindhu, they extended it to all those living well inside India during their conquest to refer to All the people who had that particular way of life different from their own.
Its etymology finally ended when the British called all those living in the entire Indian subcontinent as Indians.
“Hindu” was just a label given to us by Persians and Mughals. That does not change the fact that the Vedas and Vedic Sanskrit have always been, are still and will be our holiest sacredmost canonical texts. So what if they were composed in the territory that is in today’s political map of Pakistan ?
I’ve also seen claims something like Sanskrit is a Pakistani language because Panini lived in an area that falls in Pakistan. Yoga is a Pakistani art because RigVed mentions it first and RigVeds was written in an area that falls in Pakistan.
Modern Egyptians do not have any cultural links with the Egyptian civilization. Similarly, Pakistanis do not have any cultural links to RigVedic peoples.
Anyway, the Vedas do mention Ganges, Himalayas, and many other geographical entities in India. However I’ll discuss it later on.