I don't buy this one bit.
Firstly, the IA has conducted comparative trails between the T-90S and Arjun Mk.1:
Arjun tank outruns, outguns Russian T-90 | Business Standard News
The Mk.1 Arjun trounced the T-90S across the board.
Secondly kickbacks have ZILCH to do with why they are still going for large numbers of the T-90S- it is a institutional matter. The IA is used to the Russian Medium weight MBTs, adopting the heavy class Arjun represents a cataclysmic operational shift for them, one that will take a LONG time to get to terms with. They need to create and let's not forget this isn't an off the shelf product from abroad where all such operational procedures, logistical chains, spares etc are known and in place and available in an instant. This is an ingenious product that is a first of its kind in India, the IA and DRDO are still learning as they go along.
It makes sense to go for the T-90S to address the Modern MBT needs for the short term as the Arjun gets up an running in the IA.
And the T-90S orders are in NO WAY cutting into the Arjun's orders mate have no doubt about that. The IA needs some 6,000+ modern MBTs by the middle/end of the next decade to counter the twin threats from Pak and China and with the phasing out of the T-55s (happened a while back) and T-72s (imminent) there is more than enough demand to go around.