The Indus Valley is not a standalone cultural zone from the rest of India. It is part of greater India (or South Asia), but its own distinct zone, not just an extension of Northern/Western India, part of some vague “Indo Aryan” speaking region as its often treated.
The problem some (mostly Hindu Nationalists) have with this concept, is they have been taught to think India=Hindu. That the Indus Valley could be at once Indian, but not Hindu, seems inconceivable, and undermines the above ideology.
The way the Indus Valley is spoken about by Hindus in these texts is distinct from how other regions are spoken about. Sure some frontier regions that have not yet been Aryanized/Hinducized are condemned for heterodoxy or being outside Aryavarta, but the Indus region is singled out as uniquely infidel, which says a lot considering the Gangetic Hindus acknowledge that they (especially the Punjabis) have already been Aryanized ethnically, they’ve just fallen away from Hinduism (referred to as “fallen Kshatriyas”).