I _think_ what's going on is that they asked for the Aadhaar card as a "Photo ID". If you read the chat, the AMZN link that the rep points to asks for a place to upload a Photo ID. The Aadhaar card does function as a Photo ID as it has your picture and name on it. So it's similar to being asked to upload your Driver's License as a photo ID. It's possible that Amazon India has a policy of asking for Photo ID for investigating any complaints about big purchases because of fraud (I could just claim that I didn't get the phone). Since this user bought a Samsung Galaxy J7 Prime, it was probably greater than the threshold where the photo ID requirement was mandated. It's also possible that this user account was flagged as a suspicious user.
Either way, the whole biometric scare in this case is nonsense.
Either way, the whole biometric scare in this case is nonsense.