The timelines do not fully match.
I thought that is what I said. To be very clear, yes, I agree, the timelines do not match. There is a possible overlap, considering that all these dates/time periods are hypothetical, of two centuries.
The possible earliest date of the incursion of steppe immigrants is commonly accepted to be 1500 BC. The IVC, on the other hand, flourished between 3300 BC and 1300 BC, or rather, its period of greatest development, judging by the condition of archaeological remains that have been studied, was between 2600 BC and 1700 BC.
So we have to consider the possibility that the period from 3300 BC to 2600 BC was a period of growth, that from 2600 BC to 1700 BC was the period of greatest and fullest development of civic architecture, and from 1700 BC to 1300 BC, it was a period of decline, and abandonment of the large urban masses for increasingly smaller and less organised, more haphazard civic existence, and even internal migration to apparently less organised surroundings.
There is little scope for the interaction of the IVC and the steppe migrants, although the possibility of violent encounters between wandering savages who had gone through a traumatic separation from their original group and city dwellers who do not seem to have had a system of waging wars of attrition internally within their civilisation or externally.
The violent references in the Rg Veda are difficult to locate; they might have referred to clashes and skirmishes down the entire passage of the two major river systems that they encountered, the Ganga-Yamuna Doab, and the Indus Valley, as well as their initial encounters, if any, from their point of entry near present-day Swat and thereafter within the flood plains of the Punjab.
We do know, genetically, that the male component of the mixed population that constitutes the ANI profile was largely marked by steppe migrant characters, while the female component seems to have been native and autochthonous. That may reflect the taking of brides and mates as war-booty by the more aggressive migrants; nothing is known for certain, and all we have are the references to violence within the Rg Veda and genetic clues.