If they are peering at all, it is not over the route servers; otherwise they'd be routing to Quad9 already. This means they'd need to agree to peer directly using bilateral peers.
I suspect they don't actually peer at NIXI, and they use the IXP ports to sell IP transit to other networks.
For right now, we don't have the additional capacity in India to bring in Airtel traffic anyway. We are working on increasing capacity in Delhi and Mumbai by the end of the year.
Once we've increased capacity, we will reach out to them and ask, but their reputation suggests they don't have a settlement-free peering policy within India; only outside India. We'll see.
-Zachary