If they really turn off their website entirely, they'd get no traffic from search engines. I assume this means they'll either keep product pages up for the robots (and then redirect you to the mobile app), or that their 40M registered users simply go directly to Flipkart when they want to buy something.
Google is indexing apps and prompting an install from the SERP when they return in-app results. I doubt that it's as effective from an SEO perspective as having a good mobile website, but it is supported.
If there description in the article is correct, I would bet on users going directly to the app to buy something. I bet a lot of discovery is word of mouth and traditional media.
I would bet on leaving the website as a catalog and nothing more. Shouldn't be too hard to dump the database to get product pages.
