Interesting question. A question that precedes this is "What is the definition of a startup?" Since you have asked it in the context of an Indian business, let's try and look at it from an Indian perspective.
The Government of India actually has come up with its own definition of a startup, which is:
"To bring uniformity in the identified enterprises, an entity shall be
considered as a 'startup'-
a) Up to five years from the date of its incorporation/registration,
b) If its turnover for any of the financial years has not exceeded Rupees 25 crore, and
c) It is working towards innovation, development, deployment or commercialization of new products,
processes or services driven by technology or intellectual property;
Provided that any such entity formed by splitting up or reconstruction of a business already in existence shall
not be considered a 'startup';"
Going by the above, Flipkart, and many other so-called startups, have ceased to be startups. However, the word is considered cool - especially in India - where people talk about the startup culture like it's the hippest thing to be a part of. They would perhaps always like to think of themselves as startups, as long as it is considered cool and fashionable to do so, and as long as they can attract talent and eyeballs. When a cooler term comes around, they might adopt it too.
Honestly, nothing wrong with it, as long as you don't read too much into all the words thrown around these days.