I fully accept that the place of Vedic culture in the broader Indian cultural context is debated. However, I was conceding the most extreme position for arguments' sake that Vedic culture is 100% indigenous to the IVC and that it forms the bedrock of Indian culture. Even with that concession, my point was to posit the irrelevance of a large scale physical "Aryan" migration (genetics) and to focus on the core question of what is "indigenous" and what is "foreign", since that is supposedly the big difference between Vedic and Islamic influences within modern Indian culture.
That goes to the heart of the indigenous v/s foreign debate. I wrote "Indian" as a sloppy shorthand for the subcontinent; we could substitute the cultural, not political, entity Bharat instead and it wouldn't change anything.
I suggest that the only way to claim indigenous status for anything is if it occurred within the same political unit at the time it happened. Not centuries later. Germans can not claim Zola and Voltaire as indigenous, even though they share some level of common culture with France; less so with Poles and Greeks. Going forward, one could arguably call future artifacts indigenous to the EU, but you can't apply this label retroactively to previous centuries.
By that logic, the claim that Vedic influences were wholly indigenous makes no sense when viewed from a East/South Indian perspective, since the IVC was not a part of a common political entity until several centuries later. These were foreign influences absorbed into their own culture over time, just as Islamic influence was absorbed. Now one can claim that Islamic influences came on the back of a military conquest, but there were military conquests within the subcontinent throughout history. When kingdoms ruled over each other, it is unrealistic to expect that there was no cultural transfer.
My aim is not to discredit Indian claims on the IVC or Vedic culture, since cultural India is a shared heritage between us all, but only to draw a parallel between the spread of Vedic and Islamic influences from a political point of view.