Indian culture and Hindu culture are two different things. Indian culture is a tree with many branches on which many fruits grow. Hindu culture is one such fruit. Those who can't tell the difference between a fruit and the tree it grows on shouldn't be teaching anyone anything.
Culture is a vast beast that has its roots in human existence itself. Religion is a small part of it. And those who try to equate religion with culture are frogs in a well who have probably never examined life through any lens other than the religious one. Perhaps their families heaped this ignorance upon them, perhaps they voluntarily jumped into this pit of muck themselves, perhaps they are just lying with political territorialism in mind. In any case, it's always disappointing to find grown men and women using the words religion and culture interchangeably. Makes it hard to take them seriously when they want to become teachers to the world.
A friend circle in a small town that gathers every week to talk about Premchand's books is a culture. A teenage group of Marvel fans that sends each other memes over WhatsApp every day is a culture. A bunch of working class professionals who obsess over Bitcoin is a culture. The global Star Trek community is a culture with many sub-cultures. So are the cultures of fashion, anime, bee-keeping, cycling, and dieting.
All these things have enabled people to come together and find meaning and purpose in their lives. In other words, they do exactly what religion does without necessitating a belief in the afterlife or magic. In ages past, perhaps religion - along with nationality and other tribal identities - was an important factor that enabled cultures. But these are no longer absolutely necessary for the creation or existence of culture.
Nowhere is it written that culture can only be religious. And no religion has any more claim to the overarching idea of culture than any other.
I referred to Indian culture as a tree when I began this post. But even that tree is not standing alone in the middle of a field. It is part of a vast forest that spans the world. The birds that live in the branches of this tree spend time on other trees and are fed by fruits from all over. No one eats one fruit their whole life (except perhaps a very small worm), much as no single human being is a product of one cultural identity alone.
The idea of religion as culture is outdated and toxic. It doesn't depict the reality of the human condition honestly. And those who fall for this misrepresentation limit themselves and those around them to entire lifetimes of ignorance.