Update:
Airtel audio engineer called me and told me that they are aware of this. He told me that Tata Sky is the reason for this decision from Airtel. He said that several customers have raised complaints saying Tata Sky is providing Dolby Digital Plus on many channels whereas Airtel is not providing it. So, they made a decision to just show the highest format supported by the Home Theater on all channels and content just to satisfy the users who raised such complaints. So, although the box is producing only 2 channel audio on stereo output, it fools the HT into thinking that it's receiving 5.1 channel input thus displaying the same. I also asked him about Amazon Prime not producing Dolby audio, and he said it must be raised with Amazon, as it's their app. He also said Netflix doesn't have such a problem on the box. If Amazon doesn't agree to resolve it from their end, he asked me to raise a complaint with Airtel and they will look into both the issues.